Adaptive Cursor Sharing Fail

Here is another example (besides the fact that Adaptive Cursor Sharing only gets evaluated during a PARSE call (still valid in 12c) and supports a maximum of 14 bind variables) I've recently come across at a client site where the default implementation of Adaptive Cursor Sharing fails to create a more suitable execution plan for different bind variable values.

Broken down to a bare minimum the query was sometimes executed using non-existing values for a particular bind variable, but other times these values were existing and very popular. There were two suitable candidate indexes and one of them appeared to the optimizer more attractive in case of the "non-existing" value case. Unfortunately this index was a very bad choice for the "existing and very popular" case.

The critical point of the case is that due to other, additional filters on the same table/index the final number of rows produced by the row source operation was (almost) the same for the "non-existing" and the "existing and very popular" case, but in the latter case a large proportion of the bad index had to be scanned due to the popularity of the bind value, which was the only predicate that could be used as ACCESS predicate and the additional ones could only be applied as FILTER on the index.

So although there was a suitable histogram available on the column in question and the optimizer in principle was aware of the different popularities of values and marked the cursor as "bind sensitive" it looks like when a cursor is then monitored it just is assigned to different buckets as exposed via V$SQL_CS_HISTOGRAM, based on the rows produced by the row source. Since the operation in question didn't differ much in the final number of rows produced between the two cases (either 0 rows or 1 row) the different executions were all assigned to the same bucket - although the amount of work required to produce this single row was huge, reflected in execution time and physical / logical I/Os performed per execution.

Obviously this vastly different amount of work performed to produce a similar number of rows is not used as criteria for marking a cursor as "bind aware" and evaluate different execution plans based on ranges of bind values.

There is a view V$SQL_CS_STATISTICS that includes information about CPU_TIME and BUFFER_GETS but in 11.2.0.4 it only reflects the initial execution when starting with the "non-existing" value case, but not the slow executions afterwards. In 12.1.0.2 the view is no longer populated at all, which suggests that this information is simply not used for deciding the "bind aware" status of a cursor.

Discussing the case with Mohamed Houri, who has spent significant time on investigating the "Adaptive Cursor Sharing" feature seemed to confirm this assumption.

Here is a simple test case that allows reproducing the issue:

-- FILTER1 is highly skewed (here one very popular value -1)
create table t
as
select
        rownum as id
      , rownum as n
      , case when rownum <= 100000 then rownum else -1 end as filter1
      , rownum as filter2
      , rpad('x', 200) as filler1
      , rownum as filter3
      , rpad('x', 200) as filler2
from
        dual
connect by level <= 1e6;

-- Histogram on FILTER1 should make the sample query "bind sensitive"
exec dbms_stats.gather_table_stats(null, 't', method_opt => 'for all columns size 1 for columns size 254 filter1')

-- The "good" index, ACCESS on all three columns
-- But higher BLEVEL (3) due to FILLER1 only one value and wide - needs to be part of every branch block
create index t_idx1 on t (filter1, filler1, filter2) compress 2 /*advanced low*/;

-- The "bad" index, ACCESS only on first column, FILTER on subsequent columns
-- But lower BLEVEL (2) due to FILTER3 before FILLER1, leads to much more compact branch blocks
create index t_idx2 on t (filter1, filter3, filler1, filler2, filter2) compress 1 /*advanced low*/;

The critical point is that the good index has a higher BLEVEL than the bad index. So in case a non-existing value for FILTER1 gets used the optimizer will favor the index with the lower BLEVEL, which is for that case the key cost component:

set echo on

-- Non-existing value
-- T_IDX2 gets preferred
explain plan for
select sum(n) from t where filter1 = -999999 and filler1 = rpad('x', 200) and filter2 = 999999;

select * from table(dbms_xplan.display);

-----------------------------------------------------------------------------------------------
| Id  | Operation                            | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                     |        |     1 |   216 |     4   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE                      |        |     1 |   216 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID BATCHED| T      |     1 |   216 |     4   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN                  | T_IDX2 |     1 |       |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("FILTER1"=(-999999) AND "FILLER1"='x

                                                                                         ' AND
              "FILTER2"=999999)
       filter("FILTER2"=999999 AND "FILLER1"='x

                                                                                      ')
                                                                                     
-- Non-existing value
-- T_IDX1 has a higher cost
explain plan for
select /*+ index(t t_idx1) */ sum(n) from t where filter1 = -999999 and filler1 = rpad('x', 200) and filter2 = 999999;

select * from table(dbms_xplan.display);

-----------------------------------------------------------------------------------------------
| Id  | Operation                            | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                     |        |     1 |   216 |     5   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE                      |        |     1 |   216 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID BATCHED| T      |     1 |   216 |     5   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN                  | T_IDX1 |     1 |       |     4   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("FILTER1"=(-999999) AND "FILLER1"='x

                                                                                         ' AND
              "FILTER2"=999999)
              
-- Existing and very popular value
-- T_IDX1 gets preferred
explain plan for
select sum(n) from t where filter1 = -1 and filler1 = rpad('x', 200) and filter2 = 999999;

select * from table(dbms_xplan.display);

-----------------------------------------------------------------------------------------------
| Id  | Operation                            | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                     |        |     1 |   216 |     5   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE                      |        |     1 |   216 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID BATCHED| T      |     1 |   216 |     5   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN                  | T_IDX1 |     1 |       |     4   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("FILTER1"=(-1) AND "FILLER1"='x

                                                                                    ' AND
              "FILTER2"=999999)


-- Existing and very popular value
-- T_IDX2 is a bad idea, correctly reflected in the cost estimate
explain plan for
select /*+ index(t t_idx2) */ sum(n) from t where filter1 = -1 and filler1 = rpad('x', 200) and filter2 = 999999;

select * from table(dbms_xplan.display);

-----------------------------------------------------------------------------------------------
| Id  | Operation                            | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                     |        |     1 |   216 | 58269   (1)| 00:00:03 |
|   1 |  SORT AGGREGATE                      |        |     1 |   216 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID BATCHED| T      |     1 |   216 | 58269   (1)| 00:00:03 |
|*  3 |    INDEX RANGE SCAN                  | T_IDX2 |     1 |       | 58268   (1)| 00:00:03 |
-----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - access("FILTER1"=(-1) AND "FILLER1"='x

                                                                                    ' AND
              "FILTER2"=999999)
       filter("FILTER2"=999999 AND "FILLER1"='x

                                                                                      ')

So when optimizing for the two different cases the optimizer does the right thing and understands that for the case "existing and very popular" T_IDX2 is a bad choice. It's also obvious from the "Predicate Information" section that the index T_IDX2 only can use FILTER1 as ACCESS predicate.

But when using bind variables the different cases are not recognized and the bad index is used for both cases when the optimization is based on the "non-existing value" case:

-- Default behaviour, although being BIND_SENSITIVE thanks to the histogram, no ACS kicks in
-- The V$SQL_CS_HISTOGRAM shows all executions being in bucket 0, so according to BIND_SENSITIVE monitoring no need for action
set echo on timing on time on

alter session set statistics_level = all;

variable n number

exec :n := -999999

declare
  res number;
begin
  select sum(n) into res from t where filter1 = :n and filler1 = rpad('x', 200) and filter2 = 999999;
end;
/

exec :n := -1

declare
  res number;
begin
  for i in 1..1000 loop
    select sum(n) into res from t where filter1 = :n and filler1 = rpad('x', 200) and filter2 = 999999;
  end loop;
end;
/

SQL> alter session set statistics_level = all;

Session altered.

Elapsed: 00:00:00.03
SQL>
SQL> variable n number
SQL>
SQL> exec :n := -999999

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.03
SQL>
SQL> declare
  2    res number;
  3  begin
  4    select sum(n) into res from t where filter1 = :n and filler1 = rpad('x', 200) and filter2 = 999999;
  5  end;
  6  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.28
SQL>
SQL> exec :n := -1

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.04
SQL>
SQL> declare
  2    res number;
  3  begin
  4    for i in 1..1000 loop
  5      select sum(n) into res from t where filter1 = :n and filler1 = rpad('x', 200) and filter2 = 999999;
  6    end loop;
  7  end;
  8  /

PL/SQL procedure successfully completed.

Elapsed: 00:04:48.64
SQL>

-- With BIND_AWARE a second child cursors gets generated and used on second parse, interesting
set echo on timing on time on

alter session set statistics_level = all;

variable n number

exec :n := -999999

declare
  res number;
begin
  select /*+ bind_aware */ sum(n) into res from t where filter1 = :n and filler1 = rpad('x', 200) and filter2 = 999999;
end;
/

exec :n := -1

declare
  res number;
begin
  for i in 1..1000 loop
    select /*+ bind_aware */ sum(n) into res from t where filter1 = :n and filler1 = rpad('x', 200) and filter2 = 999999;
  end loop;
end;
/

SQL> alter session set statistics_level = all;

Session altered.

Elapsed: 00:00:00.03
SQL>
SQL> variable n number
SQL>
SQL> exec :n := -999999

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.03
SQL>
SQL> declare
  2    res number;
  3  begin
  4    select /*+ bind_aware */ sum(n) into res from t where filter1 = :n and filler1 = rpad('x', 200) and filter2 = 999999;
  5  end;
  6  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.04
SQL>
SQL> exec :n := -1

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.03
SQL>
SQL> declare
  2    res number;
  3  begin
  4    for i in 1..1000 loop
  5      select /*+ bind_aware */ sum(n) into res from t where filter1 = :n and filler1 = rpad('x', 200) and filter2 = 999999;
  6    end loop;
  7  end;
  8  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.20
SQL>

-- Or hinting for a fixed plan helps, too
set echo on timing on time on

alter session set statistics_level = all;

variable n number

exec :n := -999999

declare
  res number;
begin
  select /*+ index(t t_idx1) */ sum(n) into res from t where filter1 = :n and filler1 = rpad('x', 200) and filter2 = 999999;
end;
/

exec :n := -1

declare
  res number;
begin
  for i in 1..1000 loop
    select /*+ index(t t_idx1) */ sum(n) into res from t where filter1 = :n and filler1 = rpad('x', 200) and filter2 = 999999;
  end loop;
end;
/

Executing the unhinted, default case leads to a very inefficient execution of the thousand executions using the popular value (and the actual table / index at the client site was much larger than this example and the query got executed very frequently) - compare that to the execution time for the other two variants, either using the BIND_AWARE hint or simply requesting the good index.

So when requesting to mark the cursor immediately BIND_AWARE via the corresponding hint (only possible if the cursor qualifies to be bind sensitive) Oracle happily generates a second child cursor at the second PARSE call with a different, more efficient execution plan for the popular value, because when a cursor is marked bind aware a completely different algorithm gets used that is based on range of values as exposed via V$SQL_CS_SELECTIVITY, and since the different values are falling into different ranges of values a new plan gets generated that is different from the previous one and hence gets used from then on for those (range of) values.

Volatile Data, Dynamic Sampling And Shared Cursors

For the next couple of weeks I'll be picking up various random notes I've made during the sessions that I've attended at OOW. This particular topic was also a problem discussed recently at one of my clients, so it's certainly worth to be published here.

In one of the optimizer related sessions it was mentioned that for highly volatile data - for example often found in Global Temporary Tables (GTT) - it's recommended to use Dynamic Sampling rather than attempting to gather statistics. In particular for GTTs gathering statistics is problematic because the statistics are used globally and shared across all sessions. But GTTs could have a completely different data volume and distribution per session so sharing the statistics doesn't make sense in such scenarios.

So using Dynamic Sampling sounds like a reasonable advice and it probably is in many such cases.

However, there is still a potential problem even when resorting to Dynamic Sampling. If the cursors based on Dynamic Sampling get shared between sessions then they won't be re-optimized even if a GTT in one session is completely different from the one of the session that created the shared cursor previously.

So you can still end up with shared cursors and execution plans that are inappropriate to share across the different sessions. Using Dynamic Sampling doesn't address this issue. It addresses the issue if the cursors do not get shared, for example if they use literals and these literals differ so that different cursors will be generated based on the text matching.

Here is a simple test case that demonstrates the problem:

drop view v_gtt_dyn;

drop table gtt_dyn;

-- Create a Global Temporary Table with an index on it
create global temporary table gtt_dyn (
id number not null
, vc1 varchar2(100)
, filler varchar2(255)
)
on commit preserve rows
;

create index gtt_dyn_idx on gtt_dyn (id);

-- Create a simple view - it will become obvious later
-- why this has been used
create or replace view v_gtt_dyn as select * from gtt_dyn;

-- Run in Session 1
set echo on timing on

-- Unique value in ID column
insert into gtt_dyn
select
rownum as id
, rpad('x', 100) as vc1
, rpad('y', 255) as filler
from
dual
connect by level <= 10000
;

commit;

alter session set statistics_level = all;

alter session set optimizer_dynamic_sampling = 2;

select * from v_gtt_dyn where id = 10 and rownum > 1;

set linesize 200 pagesize 0 trimspool on tab off

select * from table(dbms_xplan.display_cursor(null, null, 'ALLSTATS LAST'));

-- Run in Session 2
set echo on timing on

-- Single value in ID column
insert into gtt_dyn
select
10 as id
, rpad('x', 100) as vc1
, rpad('y', 255) as filler
from
dual
connect by level <= 10000
;

commit;

alter session set statistics_level = all;

alter session set optimizer_dynamic_sampling = 2;

select * from v_gtt_dyn where id = 10 and rownum > 1;

set linesize 200 pagesize 0 trimspool on tab off

select * from table(dbms_xplan.display_cursor(null, null, 'ALLSTATS LAST'));

Now this is probably an extreme case of data distribution differences but if you run it you'll see it makes the point obvious: In the second session the data distribution of the GTT is completely different, and although there are no statistics gathered on the GTT and hence Dynamic Sampling gets used to arrive at an execution plan, the plan gets shared in the second session (there is only a child number 0) - but the plan is completely inappropriate for the data distribution of the GTT in the that session, you just need to look at the E-Rows and A-Rows columns of the runtime profile:

SQL> select * from v_gtt_dyn where id = 10 and rownum > 1;

no rows selected

Elapsed: 00:00:00.07
SQL>
SQL> set linesize 200 pagesize 0 trimspool on tab off
SQL>
SQL> select * from table(dbms_xplan.display_cursor(null, null, 'ALLSTATS LAST'));
SQL_ID bjax3mksw1uza, child number 0
-------------------------------------
select * from v_gtt_dyn where id = 10 and rownum > 1

Plan hash value: 471827990

-------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 0 |00:00:00.08 | 1117 |
| 1 | COUNT | | 1 | | 0 |00:00:00.08 | 1117 |
|* 2 | FILTER | | 1 | | 0 |00:00:00.08 | 1117 |
| 3 | TABLE ACCESS BY INDEX ROWID| GTT_DYN | 1 | 1 | 10000 |00:00:00.06 | 1117 |
|* 4 | INDEX RANGE SCAN | GTT_DYN_IDX | 1 | 1 | 10000 |00:00:00.02 | 63 |
-------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter(ROWNUM>1)
4 - access("ID"=10)

Note
-----
- dynamic sampling used for this statement (level=2)

Imagine a more complex plan with joins and a larger data volume and this is a receipt for disaster.

If this problem cannot be addressed from application side by helping the database to generate different cursors for the different data distributions (for example by simply adding different predicates that don't change the result like 1 = 1, 2 = 2 etc.) then you might be able to handle the issue by using Virtual Private Database (VPD, aka. Row Level Security / RLS, Fine Grained Access Control / FGAC). I've already demonstrated the general approach in the past here, but in this case a slightly more sophisticated approach could make sense.

By adding the following code and RLS policy I can drive Oracle to perform a re-optimization only in those cases where it is appropriate. This limits the damage that the general approach does to the Shared Pool by generating potentially numerous child cursors unconditionally.

create or replace package pkg_rls_force_hard_parse is
function force_hard_parse (in_schema varchar2, in_object varchar2) return varchar2;
end pkg_rls_force_hard_parse;
/

create or replace package body pkg_rls_force_hard_parse is
-- Cache the count in session state
g_cnt number;

function force_hard_parse (in_schema varchar2, in_object varchar2) return varchar2
is
s_predicate varchar2(100);
begin
-- Only execute query once in session
-- Change if re-evaluation is desired
if g_cnt is null then
select
count(*)
into
g_cnt
from
gtt_dyn
where
id = 10
and rownum <= 10;
end if;

-- We end up with exactly two child cursors
-- with the desired different plans
-- These child cursors will be shared accordingly
if g_cnt > 1 then
s_predicate := '1 = 1';
else
s_predicate := '2 = 2';
end if;

return s_predicate;
end force_hard_parse;
end pkg_rls_force_hard_parse;
/

-- CONTEXT_SENSITIVE avoids re-evaluation of policy function at execution time
-- Note however that it doesn't avoid re-evaluation at parse time
exec DBMS_RLS.ADD_POLICY (USER, 'v_gtt_dyn', 'hard_parse_policy', USER, 'pkg_rls_force_hard_parse.force_hard_parse', 'select', policy_type => DBMS_RLS.CONTEXT_SENSITIVE);

Now if you repeat above exercise - ideally with SQL trace enabled to see the additional acitivity caused by the RLS policy - you'll notice that the different sessions will end up with different child cursors and execution plans based on the check made.

Now the reason why the view is in place might become obvious: A RLS policy on the base table would have lead to an infinite recursive execution of the RLS policy function due to the query performed within the function. There are other obvious options how to deal with that, for example storing the RLS policy function in a separate schema with the EXEMPT ACCESS POLICY privilege should also work.

This is the result in the second session now:

SQL> select * from v_gtt_dyn where id = 10 and rownum > 1;

no rows selected

Elapsed: 00:00:00.12
SQL>
SQL> set linesize 200 pagesize 0 trimspool on tab off
SQL>
SQL> select * from table(dbms_xplan.display_cursor(null, null, 'ALLSTATS LAST'));
SQL_ID bjax3mksw1uza, child number 1
-------------------------------------
select * from v_gtt_dyn where id = 10 and rownum > 1

Plan hash value: 424976618

-----------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 0 |00:00:00.04 | 1003 |
| 1 | COUNT | | 1 | | 0 |00:00:00.04 | 1003 |
|* 2 | FILTER | | 1 | | 0 |00:00:00.04 | 1003 |
|* 3 | TABLE ACCESS FULL| GTT_DYN | 1 | 9288 | 10000 |00:00:00.03 | 1003 |
-----------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - filter(ROWNUM>1)
3 - filter("ID"=10)

Note
-----
- dynamic sampling used for this statement (level=2)

Notice how a second child cursor has been generated and that the cardinality estimate is now much closer to the reality.

Adaptive Cursor Sharing / Cardinality Feedback

I was curious to see if recent features like Adaptive Cursor Sharing or Cardinality Feedback would be able to solve the issue when using the 11g releases.

Cardinality Feedback (introduced in 11.2) unfortunately doesn't get used in the scenario described here, because Dynamic Sampling disables Cardinality Feedback in the current implementation.

Note that the usage of bind variables also disables Cardinality Feedback for those parts of the plan affected by the bind variables - as described in the Optimizer blog post that can be found here.

So may be Adaptive Cursor Sharing (ACS, introduced in 11.1) can come to rescue in case bind variables get used.

Of course the usage of bind variables increases the probability of cursor sharing in above scenario. As already outlined in a previous note ACS is a "reactive" and "non-persistent" feature, so it will only be able to correct things that have already been going wrong at least once. Furthermore if the ACS information gets aged out of the Shared Pool again mistakes will have to be repeated to get recognized by ACS.

However it is interesting to note that I wasn't able to get ACS working in a slightly modified scenario like this (without the RLS policy in place of course):

-- Session 1
set echo on timing on

set linesize 200 pagesize 0 trimspool on tab off

insert into gtt_dyn
select
rownum as id
, rpad('x', 100) as vc1
, rpad('y', 255) as filler
from
dual
connect by level <= 10000
;

commit;

alter session set statistics_level = all;

variable n1 number

exec :n1 := 10

select * from v_gtt_dyn where id <= :n1 and rownum > 1;

select * from table(dbms_xplan.display_cursor(null, null, 'ALLSTATS LAST'));

-- Session 2
set echo on timing on

set linesize 200 pagesize 0 trimspool on tab off

insert into gtt_dyn
select
10 as id
, rpad('x', 100) as vc1
, rpad('y', 255) as filler
from
dual
connect by level <= 10000
;

commit;

alter session set statistics_level = all;

variable n1 number

exec :n1 := 10

select * from v_gtt_dyn where id <= :n1 and rownum > 1;

select * from table(dbms_xplan.display_cursor(null, null, 'ALLSTATS LAST'));

-- Second execution to allow ACS potentially kicking in
select * from v_gtt_dyn where id <= :n1 and rownum > 1;

select * from table(dbms_xplan.display_cursor(null, null, 'ALLSTATS LAST'));

There are some interesting points to notice when running this example:

1. A cursor that uses non-equal operators like above less or equal together with bind variables usually gets marked as "bind-sensitive" and will be monitored by ACS. But in the above case the cursor was not marked as bind sensitive and hence ACS didn't even bother to monitor

2. Consequently the two sessions share the single child cursor and the problem is not addressed by ACS even in subsequent executions

3. It looks like that again the usage of Dynamic Sampling disables ACS

Looking at the way ACS manages the Cursor Sharing criteria (check V$SQL_CS_SELECTIVITY for example) I see the problem that ACS probably wouldn't support the fact that the same value for the bind variable resulted in a completely different selectivity range.

May be this is an explanation why ACS is not activated for cursors that use Dynamic Sampling - ACS may only be able to cope with different bind value ranges that lead to different selectivities.

So even when using bind variables and 11g with ACS it looks like that only the RLS policy approach allows to address this issue from a database-only side. Ideally the application should be "data-aware" in such cases and help the database accordingly to arrive at reasonable execution plans by actively unsharing the cursors.

Adaptive Cursor Sharing

This post was motivated by a recent discussion I had where I mentioned that Adaptive Cursor Sharing (ACS) does not work with embedded SQL in PL/SQL, but when looking for an existing post realized that I didn't find any good and freely available examples of that limitation of ACS.

Therefore I decided to put together this post that outlines what ACS is supposed to do, its inherent limitations and finally shed some light on the specific limitation that started its life with the description that "it does not work with embedded SQL in PL/SQL".

Adaptive Cursor Sharing (ACS) has been introduced in Oracle 11g to address the potential threat of cursor sharing and bind variables - yes, you have read correctly: The usage of bind variables can be a possible threat if different values of bind variables are executed using the same execution plan, but actually would require different execution plans to prevent poor performing executions.

So ACS allows Oracle to detect such problems and selectively "unshare" cursors, which means that Oracle creates one or more additional child cursors for different (ranges) of bind values.

However, there are a couple of limitations to this strategy. Let's have a look at a working example of ACS. Therefore I run the following script against database versions 11.1.0.7 and 11.2.0.2:

--------------------------------------------------------------------------------
--
-- File name: adaptive_cursor_sharing_1.sql
--
-- Purpose: Demonstrate a simple case of adaptive cursor sharing
--
-- Author: Randolf Geist
--
-- Last tested: December 2010
--
-- Versions: 11.1.0.7
-- 11.2.0.2
--------------------------------------------------------------------------------

set echo on define on tab off

drop table t;
purge table t;
create table t as select * from dba_objects, (select rownum from dual connect by level <= 20);
create index i on t(object_id);

exec dbms_stats.gather_table_stats(user, 'T', method_opt => 'for all columns size 1')

set echo on

select count(*), min(object_id)
, max(object_id) from t;

set echo on linesize 200 heading off newpage none tab off serveroutput off

alter session set statistics_level = all;

alter system flush buffer_cache;

alter system flush shared_pool;

var x number

exec :x := 100000

-- pause

set timing on

select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
);

column prev_sql_id new_value sql_id noprint
column sid new_value session_id noprint

select
prev_sql_id
, sid
from
v$session
where
sid = sys_context('userenv', 'sid');

select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));

exec :x := 1

select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
);

select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

exec :x := 1

select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
);

select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

set heading on

select
sql_id
, child_number
, executions
, parse_calls
, buffer_gets
, is_bind_sensitive
, is_bind_aware
from
v$sql
where
sql_id = '&sql_id';

and I get the following result from 11.2.0.2 (and 11.1.0.7):

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL>
SQL> drop table t;

Table dropped.

SQL> purge table t;

Table purged.

SQL> create table t as select * from dba_objects, (select rownum from dual connect by level <= 20);

Table created.

SQL> create index i on t(object_id);

Index created.

SQL>
SQL> exec dbms_stats.gather_table_stats(user, 'T', method_opt => 'for all columns size 1')

PL/SQL procedure successfully completed.

SQL>
SQL> set echo on
SQL>
SQL> select count(*), min(object_id)
2 , max(object_id) from t;

COUNT(*) MIN(OBJECT_ID) MAX(OBJECT_ID)
---------- -------------- --------------
1457480 2 75795

SQL>
SQL> set echo on linesize 200 heading off newpage none tab off serveroutput off
SQL>
SQL> alter session set statistics_level = all;

Session altered.

SQL>
SQL> alter system flush buffer_cache;

System altered.

SQL>
SQL> alter system flush shared_pool;

System altered.

SQL>
SQL> var x number
SQL>
SQL> exec :x := 100000

PL/SQL procedure successfully completed.

SQL>
SQL> -- pause
SQL>
SQL> set timing on
SQL>
SQL> select
2 sum(row_num)
3 from
4 (
5 select
6 row_number() over (partition by object_type order by object_name) as row_num
7 , t.*
8 from
9 t
10 where
11 object_id > :x
12 );


Elapsed: 00:00:00.09
SQL>
SQL> column prev_sql_id new_value sql_id noprint
SQL> column sid new_value session_id noprint
SQL>
SQL> select
2 prev_sql_id
3 , sid
4 from
5 v$session
6 where
7 sid = sys_context('userenv', 'sid');


Elapsed: 00:00:00.15
SQL>
SQL> select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));
old 1: select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'))
new 1: select * from table(dbms_xplan.display_cursor('fmbq5ytmh0hng',null,'ALLSTATS LAST'))
SQL_ID fmbq5ytmh0hng, child number 0
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 1399240396

-------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
-------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 3 | 3 | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 3 | 3 | | | |
| 2 | VIEW | | 1 | 13 | 0 |00:00:00.01 | 3 | 3 | | | |
| 3 | WINDOW SORT | | 1 | 13 | 0 |00:00:00.01 | 3 | 3 | 1024 | 1024 | |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 13 | 0 |00:00:00.01 | 3 | 3 | | | |
|* 5 | INDEX RANGE SCAN | I | 1 | 13 | 0 |00:00:00.01 | 3 | 3 | | | |
-------------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)


25 rows selected.

Elapsed: 00:00:01.74
SQL>
SQL> exec :x := 1

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.00
SQL>
SQL> select
2 sum(row_num)
3 from
4 (
5 select
6 row_number() over (partition by object_type order by object_name) as row_num
7 , t.*
8 from
9 t
10 where
11 object_id > :x
12 );
2.7361E+11

Elapsed: 00:00:17.12
SQL>
SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
SQL_ID fmbq5ytmh0hng, child number 0
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 1399240396

--------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | Writes | OMem | 1Mem | Used-Mem | Used-Tmp|
--------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:17.12 | 1460K| 35371 | 10532 | | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:17.12 | 1460K| 35371 | 10532 | | | | |
| 2 | VIEW | | 1 | 13 | 1457K|00:00:16.94 | 1460K| 35371 | 10532 | | | | |
| 3 | WINDOW SORT | | 1 | 13 | 1457K|00:00:16.66 | 1460K| 35371 | 10532 | 93M| 3313K| 37M (1)| 84992 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 13 | 1457K|00:00:12.01 | 1460K| 24839 | 0 | | | | |
|* 5 | INDEX RANGE SCAN | I | 1 | 13 | 1457K|00:00:01.53 | 3221 | 3220 | 0 | | | | |
--------------------------------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)


25 rows selected.

Elapsed: 00:00:00.17
SQL>
SQL> exec :x := 1

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.00
SQL>
SQL> select
2 sum(row_num)
3 from
4 (
5 select
6 row_number() over (partition by object_type order by object_name) as row_num
7 , t.*
8 from
9 t
10 where
11 object_id > :x
12 );
2.7361E+11

Elapsed: 00:00:05.10
SQL>
SQL> select * from table(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));
SQL_ID fmbq5ytmh0hng, child number 1
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 2719131525

----------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | Writes | OMem | 1Mem | Used-Mem | Used-Tmp|
----------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:05.10 | 21539 | 29966 | 8375 | | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:05.10 | 21539 | 29966 | 8375 | | | | |
| 2 | VIEW | | 1 | 1457K| 1457K|00:00:04.98 | 21539 | 29966 | 8375 | | | | |
| 3 | WINDOW SORT | | 1 | 1457K| 1457K|00:00:04.80 | 21539 | 29966 | 8375 | 74M| 2971K| 37M (1)| 67584 |
|* 4 | TABLE ACCESS FULL| T | 1 | 1457K| 1457K|00:00:01.38 | 21532 | 21528 | 0 | | | | |
----------------------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

4 - filter("OBJECT_ID">:X)


24 rows selected.

Elapsed: 00:00:00.05
SQL>
SQL> set heading on
SQL>
SQL> select
2 sql_id
3 , child_number
4 , executions
5 , parse_calls
6 , buffer_gets
7 , is_bind_sensitive
8 , is_bind_aware
9 from
10 v$sql
11 where
12 sql_id = '&sql_id';
old 12: sql_id = '&sql_id'
new 12: sql_id = 'fmbq5ytmh0hng'
SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS I I
------------- ------------ ---------- ----------- ----------- - -
fmbq5ytmh0hng 0 2 3 1460762 Y N
fmbq5ytmh0hng 1 1 0 21539 Y Y

Elapsed: 00:00:00.00
SQL>

So I intentionally use bind variables in a dangerous way here: The "object_id > :x" predicate is a potential threat - depending on the value of :x the usage of the index on OBJECT_ID can be a very good or a very bad idea.

From this example we therefore can tell a couple of things you should know about ACS:

1. ACS is not limited to the typical "skewed column data" / "histogram" case. We can see that I explicitly did not create any histograms (method_opt "for all columns size 1"), still ACS marked the cursor as "BIND_SENSITIVE" and eventually created a second child cursor with a more appropriate execution plan for the bind value, because it detected the "expression > :x" predicate.

2. However, and this one of the most important aspects, for ACS to detect the problem, it first has to go wrong at least once. As you can see from the example it was only the second execution using the "bad" bind value that created the child cursor. So depending on how "wrong" things can go - think of a more complex execution plan including multiple joins - it might already be "too late" for your application: If for example a query is not going to return in a reasonable amount of time your application might effectively be "broken" even if ACS might be able to "fix" the problem at the next execution...

3. Another important point to consider is that the ACS information is not stored persistently but only exists in the Shared Pool. This means, if the information is aged out of the Shared Pool for whatever reason, the same mistake has to be made again for ACS to detect the problem. So if your problematic cursors age out of the Shared Pool it will potentially go "wrong" again with the next execution before ACS can come for help.

From this you can see that - although ACS is a great feature - it is not a "Silver Bullet" for all possible cases. You still might need to help your database for those special cases where ACS is not able to support your application properly. How you can help your database? More on that later.

Now let's have a look at the point that ACS "does not work with embedded SQL in PL/SQL". Therefore I put together the following script that in principle does the same as the first script, but this time uses the most commonly used techniques in PL/SQL to execute SQL:

--------------------------------------------------------------------------------
--
-- File name: adaptive_cursor_sharing_2.sql
--
-- Purpose: Demonstrate adaptive cursor sharing is not working with PL/SQL when
-- optimizations are enabled, in particular holding cursors open
-- See bug 8357294
--
-- Author: Randolf Geist
--
-- Last tested: December 2010
--
-- Versions: 11.1.0.7
-- 11.2.0.2
--------------------------------------------------------------------------------

set echo on define on tab off

drop table t;
purge table t;
create table t as select * from dba_objects, (select rownum from dual connect by level <= 20);
create index i on t(object_id);

exec dbms_stats.gather_table_stats(user, 'T', method_opt => 'for all columns size 1')

set echo on

select count(*), min(object_id)
, max(object_id) from t;

set echo on linesize 200 heading on tab off serveroutput off

set timing on

-- This is a workaround that disables also the cursor optimizations in PL/SQL
-- In particular the PL/SQL hold cursor open optimization
-- Therefore the cursor gets reparsed and ACS kicks in
alter session set session_cached_cursors = 0;
--
-- Does this make any difference? No
-- alter session set plsql_optimize_level = 0;

variable sql_id varchar2(255)

-- Static SQL
alter system flush buffer_cache;

alter system flush shared_pool;

declare
x integer;
n_dummy number;
begin
for i in 1..10 loop
if i = 1 then
x := 100000;
else
x := 1;
end if;
select
sum(row_num)
into
n_dummy
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > x
);
if i = 1 then
select
prev_sql_id
into
:sql_id
from
v$session
where
sid = sys_context('userenv', 'sid');
end if;
end loop;
end;
/

select
sql_id
, child_number
, executions
, parse_calls
, buffer_gets
, is_bind_sensitive
, is_bind_aware
from
v$sql
where
sql_id = :sql_id;

set pagesize 0

select * from table(dbms_xplan.display_cursor(:sql_id,null,'+PEEKED_BINDS'));

set pagesize 9999

-- dynamic SQL with binds
alter system flush buffer_cache;

alter system flush shared_pool;

declare
x integer;
n_dummy number;
begin
for i in 1..10 loop
if i = 1 then
x := 100000;
else
x := 1;
end if;
execute immediate '
select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
)' into n_dummy using x;
if i = 1 then
select
prev_sql_id
into
:sql_id
from
v$session
where
sid = sys_context('userenv', 'sid');
end if;
end loop;
end;
/

select
sql_id
, child_number
, executions
, parse_calls
, buffer_gets
, is_bind_sensitive
, is_bind_aware
from
v$sql
where
sql_id = :sql_id;

set pagesize 0

select * from table(dbms_xplan.display_cursor(:sql_id,null,'+PEEKED_BINDS'));

set pagesize 9999

-- dynamic SQL with literals
alter system flush buffer_cache;

alter system flush shared_pool;

declare
x integer;
n_dummy number;
begin
for i in 1..10 loop
if i = 1 then
x := 100000;
else
x := 1;
end if;
execute immediate '
select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > ' || x || '
)' into n_dummy;
if i = 1 then
select
prev_sql_id
into
:sql_id
from
v$session
where
sid = sys_context('userenv', 'sid');
end if;
end loop;
end;
/

select
b.sql_id
, b.child_number
, b.executions
, b.parse_calls
, b.buffer_gets
, b.is_bind_sensitive
, b.is_bind_aware
from
v$sql a, v$sql b
where
a.force_matching_signature = b.force_matching_signature
and a.sql_id = :sql_id;

set pagesize 0

select
c.*
from
v$sql a
, v$sql b
, table(dbms_xplan.display_cursor(b.sql_id)) c
where
a.force_matching_signature = b.force_matching_signature
and a.sql_id = :sql_id;

set pagesize 9999

-- dynamic SQL with binds + OPEN
alter system flush buffer_cache;

alter system flush shared_pool;

declare
x integer;
n_dummy number;
c sys_refcursor;
begin
for i in 1..10 loop
if i = 1 then
x := 100000;
else
x := 1;
end if;
open c for '
select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
)' using x;
fetch c into n_dummy;
close c;
if i = 1 then
select
prev_sql_id
into
:sql_id
from
v$session
where
sid = sys_context('userenv', 'sid');
end if;
end loop;
end;
/

select
sql_id
, child_number
, executions
, parse_calls
, buffer_gets
, is_bind_sensitive
, is_bind_aware
from
v$sql
where
sql_id = :sql_id;

set pagesize 0

select * from table(dbms_xplan.display_cursor(:sql_id,null,'+PEEKED_BINDS'));

set pagesize 9999

So this script executes the same query as used in the first script ten times in a row, again deliberately using the same bind values in the same order: The first execution can benefit from the index, all following executions would better use a full table scan.

These are the different techniques used:

1. Static embedded SQL which are sometimes in PL/SQL are also called "implicit cursors". They should exhibit the same characteristics as explicitly declared cursors, which I haven't included in this script.

2. Dynamic SQL using EXECUTE IMMEDIATE and binds, so the cursor can still be shared

3. Dynamic SQL using EXECUTE IMMEDIATE and literals, which leads to different SQLs and therefore different parent cursors with potentially different executions plans since both will be parsed / optimized independently from each other

4. Dynamic SQL using OPEN / FETCH / CLOSE and binds, so the cursor again can be shared

And this is the result that I get from 11.2.0.2 (11.1.0.7 showed the same):

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL>
SQL> drop table t;

Table dropped.

SQL> purge table t;

Table purged.

SQL> create table t as select * from dba_objects, (select rownum from dual connect by level <= 20);

Table created.

SQL> create index i on t(object_id);

Index created.

SQL>
SQL> exec dbms_stats.gather_table_stats(user, 'T', method_opt => 'for all columns size 1')

PL/SQL procedure successfully completed.

SQL>
SQL> set echo on
SQL>
SQL> select count(*), min(object_id)
2 , max(object_id) from t;

COUNT(*) MIN(OBJECT_ID) MAX(OBJECT_ID)
---------- -------------- --------------
1457480 2 75819

SQL>
SQL> set echo on linesize 200 heading on tab off serveroutput off
SQL>
SQL> set timing on
SQL>
SQL> -- This is a workaround that disables also the cursor optimizations in PL/SQL
SQL> -- In particular the PL/SQL hold cursor open optimization
SQL> -- Therefore the cursor gets reparsed and ACS kicks in
SQL> -- alter session set session_cached_cursors = 0;
SQL> --
SQL> -- Does this make any difference? No
SQL> -- alter session set plsql_optimize_level = 0;

Session altered.

Elapsed: 00:00:00.00
SQL>
SQL> variable sql_id varchar2(255)
SQL>
SQL> -- Static SQL
SQL> alter system flush buffer_cache;

System altered.

Elapsed: 00:00:00.03
SQL>
SQL> alter system flush shared_pool;

System altered.

Elapsed: 00:00:00.63
SQL>
SQL> declare
2 x integer;
3 n_dummy number;
4 begin
5 for i in 1..10 loop
6 if i = 1 then
7 x := 100000;
8 else
9 x := 1;
10 end if;
11 select
12 sum(row_num)
13 into
14 n_dummy
15 from
16 (
17 select
18 row_number() over (partition by object_type order by object_name) as row_num
19 , t.*
20 from
21 t
22 where
23 object_id > x
24 );
25 if i = 1 then
26 select
27 prev_sql_id
28 into
29 :sql_id
30 from
31 v$session
32 where
33 sid = sys_context('userenv', 'sid');
34 end if;
35 end loop;
36 end;
37 /

PL/SQL procedure successfully completed.

Elapsed: 00:02:50.68
SQL>
SQL> select
2 sql_id
3 , child_number
4 , executions
5 , parse_calls
6 , buffer_gets
7 , is_bind_sensitive
8 , is_bind_aware
9 from
10 v$sql
11 where
12 sql_id = :sql_id;

SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS I I
------------- ------------ ---------- ----------- ----------- - -
g89tq3q521rbg 0 10 1 13146414 Y N

Elapsed: 00:00:00.17
SQL>
SQL> set pagesize 0
SQL>
SQL> select * from table(dbms_xplan.display_cursor(:sql_id,null,'+PEEKED_BINDS'));
SQL_ID g89tq3q521rbg, child number 0
-------------------------------------
SELECT SUM(ROW_NUM) FROM ( SELECT ROW_NUMBER() OVER (PARTITION BY
OBJECT_TYPE ORDER BY OBJECT_NAME) AS ROW_NUM , T.* FROM T WHERE
OBJECT_ID > :B1 )

Plan hash value: 1399240396

---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 18 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | VIEW | | 13 | 169 | 18 (6)| 00:00:01 |
| 3 | WINDOW SORT | | 13 | 507 | 18 (6)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 13 | 507 | 17 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | I | 13 | | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Peeked Binds (identified by position):
--------------------------------------

1 - :B1 (NUMBER): 100000

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:B1)


29 rows selected.

Elapsed: 00:00:01.86
SQL>
SQL> set pagesize 9999
SQL>
SQL> -- dynamic SQL with binds
SQL> alter system flush buffer_cache;

System altered.

Elapsed: 00:00:00.09
SQL>
SQL> alter system flush shared_pool;

System altered.

Elapsed: 00:00:00.84
SQL>
SQL> declare
2 x integer;
3 n_dummy number;
4 begin
5 for i in 1..10 loop
6 if i = 1 then
7 x := 100000;
8 else
9 x := 1;
10 end if;
11 execute immediate '
12 select
13 sum(row_num)
14 from
15 (
16 select
17 row_number() over (partition by object_type order by object_name) as row_num
18 , t.*
19 from
20 t
21 where
22 object_id > :x
23 )' into n_dummy using x;
24 if i = 1 then
25 select
26 prev_sql_id
27 into
28 :sql_id
29 from
30 v$session
31 where
32 sid = sys_context('userenv', 'sid');
33 end if;
34 end loop;
35 end;
36 /

PL/SQL procedure successfully completed.

Elapsed: 00:03:17.26
SQL>
SQL> select
2 sql_id
3 , child_number
4 , executions
5 , parse_calls
6 , buffer_gets
7 , is_bind_sensitive
8 , is_bind_aware
9 from
10 v$sql
11 where
12 sql_id = :sql_id;

SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS I I
------------- ------------ ---------- ----------- ----------- - -
3snga4x0swy99 0 10 1 13146434 Y N

Elapsed: 00:00:00.14
SQL>
SQL> set pagesize 0
SQL>
SQL> select * from table(dbms_xplan.display_cursor(:sql_id,null,'+PEEKED_BINDS'));
SQL_ID 3snga4x0swy99, child number 0
-------------------------------------
select sum(row_num) from (
select row_number() over (partition by object_type
order by object_name) as row_num , t.*
from t where
object_id > :x )

Plan hash value: 1399240396

---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 18 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | VIEW | | 13 | 169 | 18 (6)| 00:00:01 |
| 3 | WINDOW SORT | | 13 | 507 | 18 (6)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 13 | 507 | 17 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | I | 13 | | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Peeked Binds (identified by position):
--------------------------------------

1 - :X (NUMBER): 100000

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)


31 rows selected.

Elapsed: 00:00:01.35
SQL>
SQL> set pagesize 9999
SQL>
SQL> -- dynamic SQL with literals
SQL> alter system flush buffer_cache;

System altered.

Elapsed: 00:00:00.05
SQL>
SQL> alter system flush shared_pool;

System altered.

Elapsed: 00:00:00.66
SQL>
SQL> declare
2 x integer;
3 n_dummy number;
4 begin
5 for i in 1..10 loop
6 if i = 1 then
7 x := 100000;
8 else
9 x := 1;
10 end if;
11 execute immediate '
12 select
13 sum(row_num)
14 from
15 (
16 select
17 row_number() over (partition by object_type order by object_name) as row_num
18 , t.*
19 from
20 t
21 where
22 object_id > ' || x || '
23 )' into n_dummy;
24 if i = 1 then
25 select
26 prev_sql_id
27 into
28 :sql_id
29 from
30 v$session
31 where
32 sid = sys_context('userenv', 'sid');
33 end if;
34 end loop;
35 end;
36 /

PL/SQL procedure successfully completed.

Elapsed: 00:00:52.29
SQL>
SQL> select
2 b.sql_id
3 , b.child_number
4 , b.executions
5 , b.parse_calls
6 , b.buffer_gets
7 , b.is_bind_sensitive
8 , b.is_bind_aware
9 from
10 v$sql a, v$sql b
11 where
12 a.force_matching_signature = b.force_matching_signature
13 and a.sql_id = :sql_id;

SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS I I
------------- ------------ ---------- ----------- ----------- - -
2tgumwrfx40bu 0 1 1 53 N N
03zrk5tr7zsn4 0 9 1 193851 N N

Elapsed: 00:00:00.10
SQL>
SQL> set pagesize 0
SQL>
SQL> select
2 c.*
3 from
4 v$sql a
5 , v$sql b
6 , table(dbms_xplan.display_cursor(b.sql_id)) c
7 where
8 a.force_matching_signature = b.force_matching_signature
9 and a.sql_id = :sql_id;
SQL_ID 2tgumwrfx40bu, child number 0
-------------------------------------
select sum(row_num) from (
select row_number() over (partition by object_type
order by object_name) as row_num , t.*
from t where
object_id > 100000 )

Plan hash value: 1399240396

---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 18 (100)| |
| 1 | SORT AGGREGATE | | 1 | 15 | | |
| 2 | VIEW | | 13 | 195 | 18 (6)| 00:00:01 |
| 3 | WINDOW SORT | | 13 | 507 | 18 (6)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 13 | 507 | 17 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | I | 13 | | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">100000)

SQL_ID 03zrk5tr7zsn4, child number 0
-------------------------------------
select sum(row_num) from (
select row_number() over (partition by object_type
order by object_name) as row_num , t.*
from t where
object_id > 1 )

Plan hash value: 2719131525

-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | | 20588 (100)| |
| 1 | SORT AGGREGATE | | 1 | 15 | | | |
| 2 | VIEW | | 1457K| 20M| | 20588 (1)| 00:04:08 |
| 3 | WINDOW SORT | | 1457K| 54M| 72M| 20588 (1)| 00:04:08 |
|* 4 | TABLE ACCESS FULL| T | 1457K| 54M| | 5894 (2)| 00:01:11 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

4 - filter("OBJECT_ID">1)


51 rows selected.

Elapsed: 00:00:01.24
SQL>
SQL> set pagesize 9999
SQL>
SQL> -- dynamic SQL with binds + OPEN
SQL> alter system flush buffer_cache;

System altered.

Elapsed: 00:00:00.04
SQL>
SQL> alter system flush shared_pool;

System altered.

Elapsed: 00:00:00.66
SQL>
SQL> declare
2 x integer;
3 n_dummy number;
4 c sys_refcursor;
5 begin
6 for i in 1..10 loop
7 if i = 1 then
8 x := 100000;
9 else
10 x := 1;
11 end if;
12 open c for '
13 select
14 sum(row_num)
15 from
16 (
17 select
18 row_number() over (partition by object_type order by object_name) as row_num
19 , t.*
20 from
21 t
22 where
23 object_id > :x
24 )' using x;
25 fetch c into n_dummy;
26 close c;
27 if i = 1 then
28 select
29 prev_sql_id
30 into
31 :sql_id
32 from
33 v$session
34 where
35 sid = sys_context('userenv', 'sid');
36 end if;
37 end loop;
38 end;
39 /

PL/SQL procedure successfully completed.

Elapsed: 00:01:12.12
SQL>
SQL> select
2 sql_id
3 , child_number
4 , executions
5 , parse_calls
6 , buffer_gets
7 , is_bind_sensitive
8 , is_bind_aware
9 from
10 v$sql
11 where
12 sql_id = :sql_id;

SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS I I
------------- ------------ ---------- ----------- ----------- - -
3snga4x0swy99 0 2 3 1460762 Y N
3snga4x0swy99 1 8 7 172312 Y Y

Elapsed: 00:00:00.16
SQL>
SQL> set pagesize 0
SQL>
SQL> select * from table(dbms_xplan.display_cursor(:sql_id,null,'+PEEKED_BINDS'));
SQL_ID 3snga4x0swy99, child number 0
-------------------------------------
select sum(row_num) from (
select row_number() over (partition by object_type
order by object_name) as row_num , t.*
from t where
object_id > :x )

Plan hash value: 1399240396

---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 18 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | VIEW | | 13 | 169 | 18 (6)| 00:00:01 |
| 3 | WINDOW SORT | | 13 | 507 | 18 (6)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 13 | 507 | 17 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | I | 13 | | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Peeked Binds (identified by position):
--------------------------------------

1 - :X (NUMBER): 100000

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)

SQL_ID 3snga4x0swy99, child number 1
-------------------------------------
select sum(row_num) from (
select row_number() over (partition by object_type
order by object_name) as row_num , t.*
from t where
object_id > :x )

Plan hash value: 2719131525

-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | | 20588 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | | |
| 2 | VIEW | | 1457K| 18M| | 20588 (1)| 00:04:08 |
| 3 | WINDOW SORT | | 1457K| 54M| 72M| 20588 (1)| 00:04:08 |
|* 4 | TABLE ACCESS FULL| T | 1457K| 54M| | 5894 (2)| 00:01:11 |
-------------------------------------------------------------------------------------

Peeked Binds (identified by position):
--------------------------------------

1 - :X (NUMBER): 1

Predicate Information (identified by operation id):
---------------------------------------------------

4 - filter("OBJECT_ID">:X)


61 rows selected.

Elapsed: 00:00:01.36
SQL>
SQL> set pagesize 9999
SQL>

So this script shows some interesting results:

1. It can be seen that ACS does not seem to work with several of the used variations: Both the static and the dynamic SQL using EXECUTE IMMEDIATE and binds do not create a second child cursor and re-use the same execution plan for all ten executions. Although the cursor has been marked as BIND_SENSITIVE, it has not been marked as BIND_AWARE.

Both execution variants also share the fact that there seems to be a PL/SQL optimization in place that allows to keep the cursors open - there is only a single PARSE call recorded in V$SQL for this SQL statement, although we loop through the same code ten times and therefore should in principle see as many parse as execution calls.

You would even see the same optimization in place when explicit cursors were used with explicit OPEN / FETCH / CLOSE calls: Albeit the explicit CLOSE you would still see only a single PARSE call for the statement - PL/SQL holds the cursor open for maximum performance and scalibility.

2. Another interesting side note is that the new feature "Cardinality Feedback" that has been introduced with Oracle 11.2 seems also to be not working in these examples, but this is definitely something for another post.

"Cardinality Feedback" allows Oracle to compare the cardinality estimates of the cost-based optimizer with the actual cardinalities at execution time and perform "instant tuning" by automatically adding OPT_ESTIMATE and other hints to the statement to correct the cardinality estimates and do a re-optimization of the "tuned" statement. Again this information only resides in the Shared Pool and will be potentially aged out like the ACS information.

3. The clear winner in terms of both elapsed and performed logical I/O is the third variant that creates two different cursors using literals. For both cursors the optimal execution plan has been found and nothing had to go "wrong" first to be fixed afterwards, therefore the total elapsed time and number of buffer gets is significantly lower than in the other cases. This gives us already a hint regarding the point "How can we help the database in such cases?"

Note also that although the number of buffer gets is significantly lower compared to the other cases, the elapsed time is not that much different, which is another example that buffer gets are not necessarily a good indicator for the overall work performed. In this particular case a lot of time is spent on writing and reading from the temporary tablespace for the WINDOW SORT operation, and since this operation has more or less to do the same work in all cases where the whole table needs to read in whatever efficient or inefficient way, the overall runtime does not differ as dramatically as the number of buffer gets. You could see this additional work in this example by checking the columns DISK_READS and DIRECT_WRITES from V$SQL and of course by checking the ASH / extended SQL trace profile.

4. The fourth variant using the dynamic SQL with OPEN obviously does not benefit from the PL/SQL optimization: V$SQL clearly shows that there have been as many PARSE calls as EXECUTE calls. Interestingly in this case ACS worked and created a second child cursor. This gives us already a hint what seems to prevent ACS from working.

The shown behaviour of ACS not working is officially documented in My Oracle Support, however not available in the Knowledge Base but only as bug number 8357294: ADAPTIVE CURSOR SHARING DOESN'T WORK FOR SQL FROM PL/SQL. It is interesting to note that the very detailed bug description does not mention a fix release or patch, but does mention that setting "session_cached_cursors" to 0 allows ACS again to work.

And indeed, when running the same script again with "session_cached_cursors" set to 0, I get this result:

Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL>
SQL> drop table t;

Table dropped.

SQL> purge table t;

Table purged.

SQL> create table t as select * from dba_objects, (select rownum from dual connect by level <= 20);

Table created.

SQL> create index i on t(object_id);

Index created.

SQL>
SQL> exec dbms_stats.gather_table_stats(user, 'T', method_opt => 'for all columns size 1')

PL/SQL procedure successfully completed.

SQL>
SQL> set echo on
SQL>
SQL> select count(*), min(object_id)
2 , max(object_id) from t;

COUNT(*) MIN(OBJECT_ID) MAX(OBJECT_ID)
---------- -------------- --------------
1457480 2 75821

SQL>
SQL> set echo on linesize 200 heading on tab off serveroutput off
SQL>
SQL> set timing on
SQL>
SQL> -- This is a workaround that disables also the cursor optimizations in PL/SQL
SQL> -- In particular the PL/SQL hold cursor open optimization
SQL> -- Therefore the cursor gets reparsed and ACS kicks in
SQL> alter session set session_cached_cursors = 0;

Session altered.

Elapsed: 00:00:00.00
SQL> --
SQL> -- Does this make any difference? No
SQL> -- alter session set plsql_optimize_level = 0;
SQL>
SQL> variable sql_id varchar2(255)
SQL>
SQL> -- Static SQL
SQL> alter system flush buffer_cache;

System altered.

Elapsed: 00:00:00.05
SQL>
SQL> alter system flush shared_pool;

System altered.

Elapsed: 00:00:00.71
SQL>
SQL> declare
2 x integer;
3 n_dummy number;
4 begin
5 for i in 1..10 loop
6 if i = 1 then
7 x := 100000;
8 else
9 x := 1;
10 end if;
11 select
12 sum(row_num)
13 into
14 n_dummy
15 from
16 (
17 select
18 row_number() over (partition by object_type order by object_name) as row_num
19 , t.*
20 from
21 t
22 where
23 object_id > x
24 );
25 if i = 1 then
26 select
27 prev_sql_id
28 into
29 :sql_id
30 from
31 v$session
32 where
33 sid = sys_context('userenv', 'sid');
34 end if;
35 end loop;
36 end;
37 /

PL/SQL procedure successfully completed.

Elapsed: 00:01:00.82
SQL>
SQL> select
2 sql_id
3 , child_number
4 , executions
5 , parse_calls
6 , buffer_gets
7 , is_bind_sensitive
8 , is_bind_aware
9 from
10 v$sql
11 where
12 sql_id = :sql_id;

SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS I I
------------- ------------ ---------- ----------- ----------- - -
g89tq3q521rbg 0 2 3 1460742 Y N
g89tq3q521rbg 1 8 7 172312 Y Y

Elapsed: 00:00:00.16
SQL>
SQL> set pagesize 0
SQL>
SQL> select * from table(dbms_xplan.display_cursor(:sql_id,null,'+PEEKED_BINDS'));
SQL_ID g89tq3q521rbg, child number 0
-------------------------------------
SELECT SUM(ROW_NUM) FROM ( SELECT ROW_NUMBER() OVER (PARTITION BY
OBJECT_TYPE ORDER BY OBJECT_NAME) AS ROW_NUM , T.* FROM T WHERE
OBJECT_ID > :B1 )

Plan hash value: 1399240396

---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 18 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | VIEW | | 13 | 169 | 18 (6)| 00:00:01 |
| 3 | WINDOW SORT | | 13 | 507 | 18 (6)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 13 | 507 | 17 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | I | 13 | | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Peeked Binds (identified by position):
--------------------------------------

1 - :B1 (NUMBER): 100000

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:B1)

SQL_ID g89tq3q521rbg, child number 1
-------------------------------------
SELECT SUM(ROW_NUM) FROM ( SELECT ROW_NUMBER() OVER (PARTITION BY
OBJECT_TYPE ORDER BY OBJECT_NAME) AS ROW_NUM , T.* FROM T WHERE
OBJECT_ID > :B1 )

Plan hash value: 2719131525

-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | | 20588 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | | |
| 2 | VIEW | | 1457K| 18M| | 20588 (1)| 00:04:08 |
| 3 | WINDOW SORT | | 1457K| 54M| 72M| 20588 (1)| 00:04:08 |
|* 4 | TABLE ACCESS FULL| T | 1457K| 54M| | 5894 (2)| 00:01:11 |
-------------------------------------------------------------------------------------

Peeked Binds (identified by position):
--------------------------------------

1 - :B1 (NUMBER): 1

Predicate Information (identified by operation id):
---------------------------------------------------

4 - filter("OBJECT_ID">:B1)


57 rows selected.

Elapsed: 00:00:01.50
SQL>
SQL> set pagesize 9999
SQL>
SQL> -- dynamic SQL with binds
SQL> alter system flush buffer_cache;

System altered.

Elapsed: 00:00:00.07
SQL>
SQL> alter system flush shared_pool;

System altered.

Elapsed: 00:00:00.70
SQL>
SQL> declare
2 x integer;
3 n_dummy number;
4 begin
5 for i in 1..10 loop
6 if i = 1 then
7 x := 100000;
8 else
9 x := 1;
10 end if;
11 execute immediate '
12 select
13 sum(row_num)
14 from
15 (
16 select
17 row_number() over (partition by object_type order by object_name) as row_num
18 , t.*
19 from
20 t
21 where
22 object_id > :x
23 )' into n_dummy using x;
24 if i = 1 then
25 select
26 prev_sql_id
27 into
28 :sql_id
29 from
30 v$session
31 where
32 sid = sys_context('userenv', 'sid');
33 end if;
34 end loop;
35 end;
36 /

PL/SQL procedure successfully completed.

Elapsed: 00:01:12.41
SQL>
SQL> select
2 sql_id
3 , child_number
4 , executions
5 , parse_calls
6 , buffer_gets
7 , is_bind_sensitive
8 , is_bind_aware
9 from
10 v$sql
11 where
12 sql_id = :sql_id;

SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS I I
------------- ------------ ---------- ----------- ----------- - -
3snga4x0swy99 0 2 3 1460762 Y N
3snga4x0swy99 1 8 7 172312 Y Y

Elapsed: 00:00:00.13
SQL>
SQL> set pagesize 0
SQL>
SQL> select * from table(dbms_xplan.display_cursor(:sql_id,null,'+PEEKED_BINDS'));
SQL_ID 3snga4x0swy99, child number 0
-------------------------------------
select sum(row_num) from (
select row_number() over (partition by object_type
order by object_name) as row_num , t.*
from t where
object_id > :x )

Plan hash value: 1399240396

---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 18 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | VIEW | | 13 | 169 | 18 (6)| 00:00:01 |
| 3 | WINDOW SORT | | 13 | 507 | 18 (6)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 13 | 507 | 17 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | I | 13 | | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Peeked Binds (identified by position):
--------------------------------------

1 - :X (NUMBER): 100000

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)

SQL_ID 3snga4x0swy99, child number 1
-------------------------------------
select sum(row_num) from (
select row_number() over (partition by object_type
order by object_name) as row_num , t.*
from t where
object_id > :x )

Plan hash value: 2719131525

-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | | 20588 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | | |
| 2 | VIEW | | 1457K| 18M| | 20588 (1)| 00:04:08 |
| 3 | WINDOW SORT | | 1457K| 54M| 72M| 20588 (1)| 00:04:08 |
|* 4 | TABLE ACCESS FULL| T | 1457K| 54M| | 5894 (2)| 00:01:11 |
-------------------------------------------------------------------------------------

Peeked Binds (identified by position):
--------------------------------------

1 - :X (NUMBER): 1

Predicate Information (identified by operation id):
---------------------------------------------------

4 - filter("OBJECT_ID">:X)


61 rows selected.

Elapsed: 00:00:01.10
SQL>
SQL> set pagesize 9999
SQL>
SQL> -- dynamic SQL with literals
SQL> alter system flush buffer_cache;

System altered.

Elapsed: 00:00:00.03
SQL>
SQL> alter system flush shared_pool;

System altered.

Elapsed: 00:00:00.69
SQL>
SQL> declare
2 x integer;
3 n_dummy number;
4 begin
5 for i in 1..10 loop
6 if i = 1 then
7 x := 100000;
8 else
9 x := 1;
10 end if;
11 execute immediate '
12 select
13 sum(row_num)
14 from
15 (
16 select
17 row_number() over (partition by object_type order by object_name) as row_num
18 , t.*
19 from
20 t
21 where
22 object_id > ' || x || '
23 )' into n_dummy;
24 if i = 1 then
25 select
26 prev_sql_id
27 into
28 :sql_id
29 from
30 v$session
31 where
32 sid = sys_context('userenv', 'sid');
33 end if;
34 end loop;
35 end;
36 /

PL/SQL procedure successfully completed.

Elapsed: 00:00:55.97
SQL>
SQL> select
2 b.sql_id
3 , b.child_number
4 , b.executions
5 , b.parse_calls
6 , b.buffer_gets
7 , b.is_bind_sensitive
8 , b.is_bind_aware
9 from
10 v$sql a, v$sql b
11 where
12 a.force_matching_signature = b.force_matching_signature
13 and a.sql_id = :sql_id;

SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS I I
------------- ------------ ---------- ----------- ----------- - -
2tgumwrfx40bu 0 1 1 53 N N
03zrk5tr7zsn4 0 9 9 193851 N N

Elapsed: 00:00:00.11
SQL>
SQL> set pagesize 0
SQL>
SQL> select
2 c.*
3 from
4 v$sql a
5 , v$sql b
6 , table(dbms_xplan.display_cursor(b.sql_id)) c
7 where
8 a.force_matching_signature = b.force_matching_signature
9 and a.sql_id = :sql_id;
SQL_ID 2tgumwrfx40bu, child number 0
-------------------------------------
select sum(row_num) from (
select row_number() over (partition by object_type
order by object_name) as row_num , t.*
from t where
object_id > 100000 )

Plan hash value: 1399240396

---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 18 (100)| |
| 1 | SORT AGGREGATE | | 1 | 15 | | |
| 2 | VIEW | | 13 | 195 | 18 (6)| 00:00:01 |
| 3 | WINDOW SORT | | 13 | 507 | 18 (6)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 13 | 507 | 17 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | I | 13 | | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">100000)

SQL_ID 03zrk5tr7zsn4, child number 0
-------------------------------------
select sum(row_num) from (
select row_number() over (partition by object_type
order by object_name) as row_num , t.*
from t where
object_id > 1 )

Plan hash value: 2719131525

-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | | 20588 (100)| |
| 1 | SORT AGGREGATE | | 1 | 15 | | | |
| 2 | VIEW | | 1457K| 20M| | 20588 (1)| 00:04:08 |
| 3 | WINDOW SORT | | 1457K| 54M| 72M| 20588 (1)| 00:04:08 |
|* 4 | TABLE ACCESS FULL| T | 1457K| 54M| | 5894 (2)| 00:01:11 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

4 - filter("OBJECT_ID">1)


51 rows selected.

Elapsed: 00:00:01.17
SQL>
SQL> set pagesize 9999
SQL>
SQL> -- dynamic SQL with binds + OPEN
SQL> alter system flush buffer_cache;

System altered.

Elapsed: 00:00:00.04
SQL>
SQL> alter system flush shared_pool;

System altered.

Elapsed: 00:00:00.73
SQL>
SQL> declare
2 x integer;
3 n_dummy number;
4 c sys_refcursor;
5 begin
6 for i in 1..10 loop
7 if i = 1 then
8 x := 100000;
9 else
10 x := 1;
11 end if;
12 open c for '
13 select
14 sum(row_num)
15 from
16 (
17 select
18 row_number() over (partition by object_type order by object_name) as row_num
19 , t.*
20 from
21 t
22 where
23 object_id > :x
24 )' using x;
25 fetch c into n_dummy;
26 close c;
27 if i = 1 then
28 select
29 prev_sql_id
30 into
31 :sql_id
32 from
33 v$session
34 where
35 sid = sys_context('userenv', 'sid');
36 end if;
37 end loop;
38 end;
39 /

PL/SQL procedure successfully completed.

Elapsed: 00:01:05.62
SQL>
SQL> select
2 sql_id
3 , child_number
4 , executions
5 , parse_calls
6 , buffer_gets
7 , is_bind_sensitive
8 , is_bind_aware
9 from
10 v$sql
11 where
12 sql_id = :sql_id;

SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS I I
------------- ------------ ---------- ----------- ----------- - -
3snga4x0swy99 0 2 3 1460762 Y N
3snga4x0swy99 1 8 7 172312 Y Y

Elapsed: 00:00:00.16
SQL>
SQL> set pagesize 0
SQL>
SQL> select * from table(dbms_xplan.display_cursor(:sql_id,null,'+PEEKED_BINDS'));
SQL_ID 3snga4x0swy99, child number 0
-------------------------------------
select sum(row_num) from (
select row_number() over (partition by object_type
order by object_name) as row_num , t.*
from t where
object_id > :x )

Plan hash value: 1399240396

---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 18 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | |
| 2 | VIEW | | 13 | 169 | 18 (6)| 00:00:01 |
| 3 | WINDOW SORT | | 13 | 507 | 18 (6)| 00:00:01 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 13 | 507 | 17 (0)| 00:00:01 |
|* 5 | INDEX RANGE SCAN | I | 13 | | 3 (0)| 00:00:01 |
---------------------------------------------------------------------------------------

Peeked Binds (identified by position):
--------------------------------------

1 - :X (NUMBER): 100000

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)

SQL_ID 3snga4x0swy99, child number 1
-------------------------------------
select sum(row_num) from (
select row_number() over (partition by object_type
order by object_name) as row_num , t.*
from t where
object_id > :x )

Plan hash value: 2719131525

-------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | | 20588 (100)| |
| 1 | SORT AGGREGATE | | 1 | 13 | | | |
| 2 | VIEW | | 1457K| 18M| | 20588 (1)| 00:04:08 |
| 3 | WINDOW SORT | | 1457K| 54M| 72M| 20588 (1)| 00:04:08 |
|* 4 | TABLE ACCESS FULL| T | 1457K| 54M| | 5894 (2)| 00:01:11 |
-------------------------------------------------------------------------------------

Peeked Binds (identified by position):
--------------------------------------

1 - :X (NUMBER): 1

Predicate Information (identified by operation id):
---------------------------------------------------

4 - filter("OBJECT_ID">:X)


61 rows selected.

Elapsed: 00:00:01.57
SQL>
SQL> set pagesize 9999
SQL>

Now in all cases where the cursor is potentially shared we can see that ACS eventually kicks in and creates a second child cursor. Notice however that this comes at a price: All cases show now that there are as many PARSE calls as executions.

So this seems to mean that disabling the session cursor cache actually disabled also the PL/SQL optimization that prevents the repeated PARSE calls by keeping the cursors open.

Remember that the session cursor cache is merely a means to make a soft PARSE call even _softer_ by avoiding the otherwise required access to the Shared Pool library cache resources because the cursor information has been cached in a private memory area of the process, but the session cursor cache does not avoid the PARSE call itself - this can only be done on application resp. client side, not on database side.

In order to understand if the session cursor cache or the "keep cursors open" optimization affects ACS we can run the following simple script:

--------------------------------------------------------------------------------
--
-- File name: adaptive_cursor_sharing_3.sql
--
-- Purpose: Check if the session cursor cache affects adaptive cursor sharing
--
-- Author: Randolf Geist
--
-- Last tested: December 2010
--
-- Versions: 11.1.0.7
-- 11.2.0.2
--------------------------------------------------------------------------------

set echo on define on tab off

drop table t;
purge table t;
create table t as select * from dba_objects, (select rownum from dual connect by level <= 20);
create index i on t(object_id);

exec dbms_stats.gather_table_stats(user, 'T', method_opt => 'for all columns size 1')

set echo on

select count(*), min(object_id)
, max(object_id) from t;

set echo on linesize 200 heading off newpage none tab off serveroutput off

alter session set statistics_level = all;

-- alter session set session_cached_cursors = 0;

alter system flush buffer_cache;

alter system flush shared_pool;

var x number

exec :x := 100000

-- pause

set timing on

select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
);

column prev_sql_id new_value sql_id noprint
column sid new_value session_id noprint

select
prev_sql_id
, sid
from
v$session
where
sid = sys_context('userenv', 'sid');

select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));

exec :x := 100000

column value new_value value_1

select
name
, value
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
);

select
name
, value - &value_1 as diff
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));

exec :x := 100000

select
name
, value
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
);

select
name
, value - &value_1 as diff
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));

exec :x := 100000

select
name
, value
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
);

select
name
, value - &value_1 as diff
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));

exec :x := 100000

select
name
, value
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
);

select
name
, value - &value_1 as diff
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));

exec :x := 1

select
name
, value
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
);

select
name
, value - &value_1 as diff
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));

exec :x := 1

select
name
, value
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
);

select
name
, value - &value_1 as diff
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));

exec :x := 1

select
name
, value
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select
sum(row_num)
from
(
select
row_number() over (partition by object_type order by object_name) as row_num
, t.*
from
t
where
object_id > :x
);

select
name
, value - &value_1 as diff
from
v$statname a
, v$sesstat b
where
a.statistic# = b.statistic#
and a.name = 'session cursor cache hits'
and b.sid = sys_context('userenv', 'sid');

select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));

set heading on

select
sql_id
, child_number
, executions
, parse_calls
, buffer_gets
, is_bind_sensitive
, is_bind_aware
from
v$sql
where
sql_id = '&sql_id';

This script simply runs the same SQL that we already know sufficiently often to ensure that it gets cached in the session cursor cache by executing it at least three times. Then in one of the next executions the "bad" bind values is used to see if the caching in the session cursor cache somehow prevents ACS from kicking in. Here is the result, this time from 11.1.0.7 (but again I got the same from 11.2.0.2):

Connected to:
Oracle Database 11g Enterprise Edition Release 11.1.0.7.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL>
SQL> drop table t;

Table dropped.

SQL> purge table t;

Table purged.

SQL> create table t as select * from dba_objects, (select rownum from dual connect by level <= 20);

Table created.

SQL> create index i on t(object_id);

Index created.

SQL>
SQL> exec dbms_stats.gather_table_stats(user, 'T', method_opt => 'for all columns size 1')

PL/SQL procedure successfully completed.

SQL>
SQL> set echo on
SQL>
SQL> select count(*), min(object_id)
2 , max(object_id) from t;

COUNT(*) MIN(OBJECT_ID) MAX(OBJECT_ID)
---------- -------------- --------------
1385560 2 71521

SQL>
SQL> set echo on linesize 200 heading off newpage none tab off serveroutput off
SQL>
SQL> alter session set statistics_level = all;

Session altered.

SQL>
SQL> -- alter session set session_cached_cursors = 0;
SQL>
SQL> alter system flush buffer_cache;

System altered.

SQL>
SQL> alter system flush shared_pool;

System altered.

SQL>
SQL> var x number
SQL>
SQL> exec :x := 100000

PL/SQL procedure successfully completed.

SQL>
SQL> -- pause
SQL>
SQL> set timing on
SQL>
SQL> select
2 sum(row_num)
3 from
4 (
5 select
6 row_number() over (partition by object_type order by object_name) as row_num
7 , t.*
8 from
9 t
10 where
11 object_id > :x
12 );


Elapsed: 00:00:00.01
SQL>
SQL> column prev_sql_id new_value sql_id noprint
SQL> column sid new_value session_id noprint
SQL>
SQL> select
2 prev_sql_id
3 , sid
4 from
5 v$session
6 where
7 sid = sys_context('userenv', 'sid');


Elapsed: 00:00:00.06
SQL>
SQL> select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));
old 1: select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'))
new 1: select * from table(dbms_xplan.display_cursor('fmbq5ytmh0hng',null,'ALLSTATS LAST'))
SQL_ID fmbq5ytmh0hng, child number 0
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 1399240396

-------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
-------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 3 | 3 | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 3 | 3 | | | |
| 2 | VIEW | | 1 | 12 | 0 |00:00:00.01 | 3 | 3 | | | |
| 3 | WINDOW SORT | | 1 | 12 | 0 |00:00:00.01 | 3 | 3 | 1024 | 1024 | |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 12 | 0 |00:00:00.01 | 3 | 3 | | | |
|* 5 | INDEX RANGE SCAN | I | 1 | 12 | 0 |00:00:00.01 | 3 | 3 | | | |
-------------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)


25 rows selected.

Elapsed: 00:00:00.46
SQL>
SQL> exec :x := 100000

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.00
SQL>
SQL> column value new_value value_1
SQL>
SQL> select
2 name
3 , value
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
session cursor cache hits 5058

Elapsed: 00:00:00.01
SQL>
SQL> select
2 sum(row_num)
3 from
4 (
5 select
6 row_number() over (partition by object_type order by object_name) as row_num
7 , t.*
8 from
9 t
10 where
11 object_id > :x
12 );


Elapsed: 00:00:00.01
SQL>
SQL> select
2 name
3 , value - &value_1 as diff
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
old 3: , value - &value_1 as diff
new 3: , value - 5058 as diff
session cursor cache hits 2

Elapsed: 00:00:00.00
SQL>
SQL> select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));
old 1: select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'))
new 1: select * from table(dbms_xplan.display_cursor('fmbq5ytmh0hng',null,'ALLSTATS LAST'))
SQL_ID fmbq5ytmh0hng, child number 0
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 1399240396

----------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 3 | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 3 | | | |
| 2 | VIEW | | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
| 3 | WINDOW SORT | | 1 | 12 | 0 |00:00:00.01 | 3 | 1024 | 1024 | |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
|* 5 | INDEX RANGE SCAN | I | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
----------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)


25 rows selected.

Elapsed: 00:00:00.10
SQL>
SQL> exec :x := 100000

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.00
SQL>
SQL> select
2 name
3 , value
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
session cursor cache hits 5063

Elapsed: 00:00:00.00
SQL>
SQL> select
2 sum(row_num)
3 from
4 (
5 select
6 row_number() over (partition by object_type order by object_name) as row_num
7 , t.*
8 from
9 t
10 where
11 object_id > :x
12 );


Elapsed: 00:00:00.01
SQL>
SQL> select
2 name
3 , value - &value_1 as diff
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
old 3: , value - &value_1 as diff
new 3: , value - 5063 as diff
session cursor cache hits 2

Elapsed: 00:00:00.00
SQL>
SQL> select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));
old 1: select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'))
new 1: select * from table(dbms_xplan.display_cursor('fmbq5ytmh0hng',null,'ALLSTATS LAST'))
SQL_ID fmbq5ytmh0hng, child number 0
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 1399240396

----------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 3 | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 3 | | | |
| 2 | VIEW | | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
| 3 | WINDOW SORT | | 1 | 12 | 0 |00:00:00.01 | 3 | 1024 | 1024 | |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
|* 5 | INDEX RANGE SCAN | I | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
----------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)


25 rows selected.

Elapsed: 00:00:00.10
SQL>
SQL> exec :x := 100000

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.01
SQL>
SQL> select
2 name
3 , value
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
session cursor cache hits 5069

Elapsed: 00:00:00.00
SQL>
SQL> select
2 sum(row_num)
3 from
4 (
5 select
6 row_number() over (partition by object_type order by object_name) as row_num
7 , t.*
8 from
9 t
10 where
11 object_id > :x
12 );


Elapsed: 00:00:00.01
SQL>
SQL> select
2 name
3 , value - &value_1 as diff
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
old 3: , value - &value_1 as diff
new 3: , value - 5069 as diff
session cursor cache hits 3

Elapsed: 00:00:00.00
SQL>
SQL> select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));
old 1: select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'))
new 1: select * from table(dbms_xplan.display_cursor('fmbq5ytmh0hng',null,'ALLSTATS LAST'))
SQL_ID fmbq5ytmh0hng, child number 0
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 1399240396

----------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 3 | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 3 | | | |
| 2 | VIEW | | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
| 3 | WINDOW SORT | | 1 | 12 | 0 |00:00:00.01 | 3 | 1024 | 1024 | |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
|* 5 | INDEX RANGE SCAN | I | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
----------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)


25 rows selected.

Elapsed: 00:00:00.11
SQL>
SQL> exec :x := 100000

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.00
SQL>
SQL> select
2 name
3 , value
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
session cursor cache hits 5080

Elapsed: 00:00:00.00
SQL>
SQL> select
2 sum(row_num)
3 from
4 (
5 select
6 row_number() over (partition by object_type order by object_name) as row_num
7 , t.*
8 from
9 t
10 where
11 object_id > :x
12 );


Elapsed: 00:00:00.01
SQL>
SQL> select
2 name
3 , value - &value_1 as diff
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
old 3: , value - &value_1 as diff
new 3: , value - 5080 as diff
session cursor cache hits 3

Elapsed: 00:00:00.00
SQL>
SQL> select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));
old 1: select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'))
new 1: select * from table(dbms_xplan.display_cursor('fmbq5ytmh0hng',null,'ALLSTATS LAST'))
SQL_ID fmbq5ytmh0hng, child number 0
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 1399240396

----------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 3 | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 3 | | | |
| 2 | VIEW | | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
| 3 | WINDOW SORT | | 1 | 12 | 0 |00:00:00.01 | 3 | 1024 | 1024 | |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
|* 5 | INDEX RANGE SCAN | I | 1 | 12 | 0 |00:00:00.01 | 3 | | | |
----------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)


25 rows selected.

Elapsed: 00:00:00.10
SQL>
SQL> exec :x := 1

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.00
SQL>
SQL> select
2 name
3 , value
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
session cursor cache hits 5090

Elapsed: 00:00:00.00
SQL>
SQL> select
2 sum(row_num)
3 from
4 (
5 select
6 row_number() over (partition by object_type order by object_name) as row_num
7 , t.*
8 from
9 t
10 where
11 object_id > :x
12 );
2.5311E+11

Elapsed: 00:00:17.12
SQL>
SQL> select
2 name
3 , value - &value_1 as diff
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
old 3: , value - &value_1 as diff
new 3: , value - 5090 as diff
session cursor cache hits 3

Elapsed: 00:00:00.00
SQL>
SQL> select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));
old 1: select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'))
new 1: select * from table(dbms_xplan.display_cursor('fmbq5ytmh0hng',null,'ALLSTATS LAST'))
SQL_ID fmbq5ytmh0hng, child number 0
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 1399240396

--------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | Writes | OMem | 1Mem | Used-Mem | Used-Tmp|
--------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:17.10 | 1388K| 44110 | 10005 | | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:17.10 | 1388K| 44110 | 10005 | | | | |
| 2 | VIEW | | 1 | 12 | 1385K|00:00:16.03 | 1388K| 44110 | 10005 | | | | |
| 3 | WINDOW SORT | | 1 | 12 | 1385K|00:00:16.03 | 1388K| 44110 | 10005 | 88M| 3227K| 55M (1)| 80896 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 12 | 1385K|00:00:09.70 | 1388K| 24154 | 0 | | | | |
|* 5 | INDEX RANGE SCAN | I | 1 | 12 | 1385K|00:00:01.39 | 3060 | 3059 | 0 | | | | |
--------------------------------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)


25 rows selected.

Elapsed: 00:00:00.11
SQL>
SQL> exec :x := 1

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.00
SQL>
SQL> select
2 name
3 , value
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
session cursor cache hits 5100

Elapsed: 00:00:00.00
SQL>
SQL> select
2 sum(row_num)
3 from
4 (
5 select
6 row_number() over (partition by object_type order by object_name) as row_num
7 , t.*
8 from
9 t
10 where
11 object_id > :x
12 );
2.5311E+11

Elapsed: 00:00:12.94
SQL>
SQL> select
2 name
3 , value - &value_1 as diff
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
old 3: , value - &value_1 as diff
new 3: , value - 5100 as diff
session cursor cache hits 3

Elapsed: 00:00:00.01
SQL>
SQL> select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));
old 1: select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'))
new 1: select * from table(dbms_xplan.display_cursor('fmbq5ytmh0hng',null,'ALLSTATS LAST'))
SQL_ID fmbq5ytmh0hng, child number 0
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 1399240396

--------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | Writes | OMem | 1Mem | Used-Mem | Used-Tmp|
--------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:12.95 | 1388K| 44001 | 10005 | | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:12.95 | 1388K| 44001 | 10005 | | | | |
| 2 | VIEW | | 1 | 12 | 1385K|00:00:12.29 | 1388K| 44001 | 10005 | | | | |
| 3 | WINDOW SORT | | 1 | 12 | 1385K|00:00:12.29 | 1388K| 44001 | 10005 | 88M| 3232K| 55M (1)| 80896 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 12 | 1385K|00:00:06.93 | 1388K| 24059 | 0 | | | | |
|* 5 | INDEX RANGE SCAN | I | 1 | 12 | 1385K|00:00:00.01 | 3060 | 3059 | 0 | | | | |
--------------------------------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)


25 rows selected.

Elapsed: 00:00:00.11
SQL>
SQL> exec :x := 1

PL/SQL procedure successfully completed.

Elapsed: 00:00:00.00
SQL>
SQL> select
2 name
3 , value
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
session cursor cache hits 5110

Elapsed: 00:00:00.01
SQL>
SQL> select
2 sum(row_num)
3 from
4 (
5 select
6 row_number() over (partition by object_type order by object_name) as row_num
7 , t.*
8 from
9 t
10 where
11 object_id > :x
12 );
2.5311E+11

Elapsed: 00:00:06.02
SQL>
SQL> select
2 name
3 , value - &value_1 as diff
4 from
5 v$statname a
6 , v$sesstat b
7 where
8 a.statistic# = b.statistic#
9 and a.name = 'session cursor cache hits'
10 and b.sid = sys_context('userenv', 'sid');
old 3: , value - &value_1 as diff
new 3: , value - 5110 as diff
session cursor cache hits 3

Elapsed: 00:00:00.01
SQL>
SQL> select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'));
old 1: select * from table(dbms_xplan.display_cursor('&sql_id',null,'ALLSTATS LAST'))
new 1: select * from table(dbms_xplan.display_cursor('fmbq5ytmh0hng',null,'ALLSTATS LAST'))
SQL_ID fmbq5ytmh0hng, child number 0
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 1399240396

--------------------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | Writes | OMem | 1Mem | Used-Mem | Used-Tmp|
--------------------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:12.95 | 1388K| 44001 | 10005 | | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:12.95 | 1388K| 44001 | 10005 | | | | |
| 2 | VIEW | | 1 | 12 | 1385K|00:00:12.29 | 1388K| 44001 | 10005 | | | | |
| 3 | WINDOW SORT | | 1 | 12 | 1385K|00:00:12.29 | 1388K| 44001 | 10005 | 88M| 3232K| 55M (1)| 80896 |
| 4 | TABLE ACCESS BY INDEX ROWID| T | 1 | 12 | 1385K|00:00:06.93 | 1388K| 24059 | 0 | | | | |
|* 5 | INDEX RANGE SCAN | I | 1 | 12 | 1385K|00:00:00.01 | 3060 | 3059 | 0 | | | | |
--------------------------------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

5 - access("OBJECT_ID">:X)

SQL_ID fmbq5ytmh0hng, child number 1
-------------------------------------
select sum(row_num) from ( select
row_number() over (partition by object_type order by object_name) as
row_num , t.* from t
where object_id > :x )

Plan hash value: 2719131525

----------------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | Writes | OMem | 1Mem | Used-Mem | Used-Tmp|
----------------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 1 |00:00:05.99 | 21058 | 34648 | 7955 | | | | |
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:05.99 | 21058 | 34648 | 7955 | | | | |
| 2 | VIEW | | 1 | 1385K| 1385K|00:00:05.74 | 21058 | 34648 | 7955 | | | | |
| 3 | WINDOW SORT | | 1 | 1385K| 1385K|00:00:04.35 | 21058 | 34648 | 7955 | 69M| 2893K| 55M (1)| 64512 |
|* 4 | TABLE ACCESS FULL| T | 1 | 1385K| 1385K|00:00:00.01 | 21054 | 21049 | 0 | | | | |
----------------------------------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

4 - filter("OBJECT_ID">:X)


49 rows selected.

Elapsed: 00:00:00.21
SQL>
SQL> set heading on
SQL>
SQL> select
2 sql_id
3 , child_number
4 , executions
5 , parse_calls
6 , buffer_gets
7 , is_bind_sensitive
8 , is_bind_aware
9 from
10 v$sql
11 where
12 sql_id = '&sql_id';
old 12: sql_id = '&sql_id'
new 12: sql_id = 'fmbq5ytmh0hng'
SQL_ID CHILD_NUMBER EXECUTIONS PARSE_CALLS BUFFER_GETS I I
------------- ------------ ---------- ----------- ----------- - -
fmbq5ytmh0hng 0 7 8 2777315 Y N
fmbq5ytmh0hng 1 1 0 21058 Y Y

Elapsed: 00:00:00.03
SQL>

So we can see that from the third execution on we seem to get a hit in the session cursor cache (increase from 2 to 3), but still ACS seems to work properly, although this time it had to go wrong twice before ACS created the second child cursor. We can also see that we still get as many PARSE calls as executions, so the session cursor cache does not avoid the parsing as outlined above.

All this seems to suggest that the ACS (and potentially the "Cardinality Feedback", too) code is only triggered by a PARSE call and not evaluated as part of an EXECUTE call.

But if this was the case, then by no means the problem would be limited to PL/SQL and its clever optimizations that try to hold cursors open to avoid repeated PARSE calls.

Actually one of the core principles that we are told to make an application as scalable as possible is to parse only once but execute many times. So let's try to simulate such a well behaving application with this simple Java code snippet that accesses the database via JDBC:

import java.sql.* ;

class JDBCQuery
{
public static void main( String args[] )
{
try
{
// Load the database driver
Class.forName( "oracle.jdbc.driver.OracleDriver" ) ;

// Get a connection to the database
Connection conn = DriverManager.getConnection("jdbc:oracle:thin:@host:port:sid",
"username", "pwd");

// Print all warnings
for( SQLWarning warn = conn.getWarnings(); warn != null; warn = warn.getNextWarning() )
{
System.out.println( "SQL Warning:" ) ;
System.out.println( "State : " + warn.getSQLState() ) ;
System.out.println( "Message: " + warn.getMessage() ) ;
System.out.println( "Error : " + warn.getErrorCode() ) ;
}

// Reset the buffer cache and shared pool
Statement ddl = conn.createStatement();
ddl.executeUpdate("alter system flush shared_pool");
ddl.executeUpdate("alter system flush buffer_cache");
ddl.close();

// Prepare the statement
String query =
"select\n" +
" sum(row_num)\n" +
"from\n" +
" (\n" +
" select\n" +
" row_number() over (partition by object_type order by object_name) as row_num\n" +
" , t.*\n" +
" from\n" +
" t\n" +
" where\n" +
" object_id > ?\n" +
" )";

PreparedStatement stmt = conn.prepareStatement(query);

System.out.println(query);

Integer i;
Integer id;

for (i = 1; i <= 10; i++)
{
if (i == 1)
{
id = 100000;
}
else
{
id = 1;
}

System.out.println( "Iteration: " + i.toString() ) ;

stmt.setInt(1, id);

long startTime = System.currentTimeMillis();

// Execute the query
ResultSet rs = stmt.executeQuery();

// Loop through the result set
while( rs.next() )
System.out.println( rs.getLong(1)) ;

// Close the result set
rs.close() ;

long endTime = System.currentTimeMillis();

float seconds = (endTime - startTime) / 1000F;

// Elapsed time
System.out.println(Float.toString(seconds) + " seconds.");
}

stmt.close() ;
conn.close() ;
}
catch( SQLException se )
{
System.out.println( "SQL Exception:" ) ;

// Loop through the SQL Exceptions
while( se != null )
{
System.out.println( "State : " + se.getSQLState() ) ;
System.out.println( "Message: " + se.getMessage() ) ;
System.out.println( "Error : " + se.getErrorCode() ) ;

se = se.getNextException() ;
}
}
catch( Exception e )
{
System.out.println( e ) ;
}
}
}

/*
Check the Shared Pool by this query:

select
sql_id
, child_number
, executions
, parse_calls
, buffer_gets
, disk_reads
, direct_writes
, is_bind_sensitive
, is_bind_aware
from
v$sql
where
sql_id = 'cq3a8ukkws0su';

*/

This (by no means elegant or clever) piece of Java code again tries to do the same as our previous examples: It executes the known query ten times but behaves well since it parses only once and executes ten times using the two different bind values as already done before.

I only tested this with the JDBC thin driver that comes with Oracle 11.2.0.2 but not with the thick driver. However, since I assume that 95%+ of all Java applications out there actually use the thin driver this probably covers the most relevant case.

The results correspond to those of the PL/SQL examples with the "keep cursors open" optimization enabled: Neither ACS nor "Cardinality Feedback" kick in, and the SQL is executed every time using the same execution plan. The SQL is only parsed once, so the problem is not really limited to PL/SQL, but to every application that tries to avoid parse calls.

Summary

ACS is a great feature, but you need to be aware of its limitations, the most important being:

- Things have to go wrong first before ACS will be able to provide a fix. Depending on how "wrong" things can go, this might mean that ACS is actually not able to provide an efficient fix for certain cases
- The ACS information can be aged out of the Shared Pool which might lead to repeated "things have to go wrong" cases
- ACS is only triggered by explicit PARSE calls, it is not activated if the application parses once and executes many times

If you have a case where you need to help (or "co-operate" as Jonathan Lewis calls it in his brilliant presentation of the same name) your database, the simplest strategy is to use literals instead of bind variables. If you can't do this due to the issues introduced by this strategy (excessive hard parses, increased CPU and Shared Pool usage, library cache contention etc.) a more complex strategy - if applicable - is to "know your data" and generate a minimum amount of cursors that try to cover the cases where a different execution plan is desired.

This means something like this in pseudo code:

- If "known value A that requires a special treatment" then
-- execute SQL with dummy hint "A" or additional non-modifying predicate "A" = "A" to make the SQL unique
- If "known value B that requires a special treatment" then
-- execute SQL with dummy hint "B" or additional non-modifying predicate "B" = "B" to make the SQL unique
- Else "in all other cases"
-- execute SQL as is

Notice that using comments to make the SQL unique might not lead to the expected result since there are cases/environments where comments are stripped prior to execution.

Update: From a comment by Sokrates below that for some reason didn't make to the comments section but also doesn't show up in the "Spam" comments of the blog control panel I see that above description "dummy hint" might need a bit more explanation.

By "dummy hint" I mean to say that you should use hint syntax rather than normal comment syntax to make the SQL unique, because the normal comment might get filtered out whereas the comment using hint syntax is supposed to be kept. By "dummy hint" I mean any invalid hint comment in order to prevent any influence on the optimizer, for example /*+ CASE_A */ rather than /* CASE_A */