Playing around with some new features added in 23c - here the removal of the restriction so far that Direct Path loads or Parallel DML prevented access to the same object within the same transaction - which meant that before you could access the same object again within the same session after manipulating it using Direct Path loads or Parallel DML the transaction had to be committed - please note that this applies only to the session performing the manipulation - other sessions outside the transaction won't notice any difference.
So sometimes if you wanted to take advantage of these features you had to split a single transaction (from a business point of view) into multiple transactions due to this technical limitation. I assume this difference in behaviour was one of the main reasons why Oracle requires you to enable Parallel DML explicitly using ALTER SESSION ENABLE PARALLEL DML on session level or from 12c on using the ENABLE_PARALLEL_DML hint on statement level.
So generally speaking it's good to see that Oracle in 23c finally lifts this long lasting restriction - now users can make use of Direct Path and / or Parallel DML operations without having to worry about behaviour changes and splitting up transactions.
Tim Hall at Oracle-Base as usual has this new feature already covered - so you can check his notes here before continuing to get an idea what this is about.
Here I want to focus on what you get when you try this new feature with 23c FREE - Parallel Execution isn't supported in 23c FREE as far as I can see, and doesn't make a lot of sense given the limitations regarding maximum number of CPU threads allowed - but Direct Path loads are something you can certainly make use of in 23c FREE.
So to get an idea here is a very simple script (similar to what Tim used to demonstrate) using Session Statistics to confirm that Direct Path writes happened:
And here is the 19c output:
SQL*Plus: Release 21.0.0.0.0 - Production on Wed Aug 16 17:14:06 2023
Version 21.3.0.0.0
Copyright (c) 1982, 2021, Oracle. All rights reserved.
Letzte erfolgreiche Anmeldezeit: Mi Aug 16 2023 17:08:15 +02:00
Verbunden mit:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.19.0.0.0
Session altered.
SQL> column value_start new_value value_start
SQL> column value_end new_value value_end
SQL> drop table /*if exists*/ t1 purge;
Table dropped.
SQL> create table t1
2 -- Looks like the restriction on the tablespace isn't implemented properly in 23c FREE!
3 --tablespace TEST_8K
4 tablespace TEST_8K_ASSM_AUTO
5 as
6 select *
7 from all_objects;
Table created.
SQL> select count(*) from t1;
COUNT(*)
----------
67771
SQL> set termout off
SQL> insert /*+ append */ into t1
2 select * from t1;
67771 rows created.
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
1329
SQL> select count(*) from t1;
select count(*) from t1
*
ERROR at line 1:
ORA-12838: cannot read/modify an object after modifying it in parallel
SQL> set termout off
SQL> insert /*+ append */ into t1
2 select * from t1;
insert /*+ append */ into t1
*
ERROR at line 1:
ORA-12838: cannot read/modify an object after modifying it in parallel
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
0
SQL> commit;
Commit complete.
SQL> select count(*) from t1;
COUNT(*)
----------
135542
SQL> set termout off
SQL> insert /*+ append NO_MULTI_APPEND */ into t1
2 select * from t1;
135542 rows created.
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
2658
SQL> select /*+ NO_MULTI_APPEND */ count(*) from t1;
select /*+ NO_MULTI_APPEND */ count(*) from t1
*
ERROR at line 1:
ORA-12838: cannot read/modify an object after modifying it in parallel
SQL> select count(*) from t1;
select count(*) from t1
*
ERROR at line 1:
ORA-12838: cannot read/modify an object after modifying it in parallel
SQL> set termout off
SQL> insert /*+ append */ into t1
2 select * from t1;
insert /*+ append */ into t1
*
ERROR at line 1:
ORA-12838: cannot read/modify an object after modifying it in parallel
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
0
SQL> commit;
Commit complete.
SQL> select count(*) from t1;
COUNT(*)
----------
271084
As expected after performing an insert in APPEND mode the same object can't be accessed within the same transaction - the error "ORA-12838: cannot read/modify an object after modifying it in parallel" is raised (which isn't very concise anyway - this is not a parallel but a direct path modification) for both query and DML attempts. Only after ending the transaction using COMMIT access is possible again.
Now when reading the 23c documentation available so far - follow this link - the lifted restriction regarding Direct Path loads / Parallel DML comes with some interesting limitations (quoting from above link):
"However, the restrictions on the above operations still apply when:
- The tables are IOT or clustered tables. (The table must be a heap table.)
- The tablespace is not under Automatic Segment Space Management (ASSM). Temporary tables are not under ASSM, so this includes them as well.
- The tablespace is of uniform extent.
The restrictions on multiple queries and DML/PDML operations as well as the restriction on multiple direct-path inserts in the same session can be reinstated when needed by including the NO_MULTI_STATEMENT hint in SQL statements."
There is no NO_MULTI_STATEMENT hint by the way - as you will see later. So when the tablespace used for the table (what about indexes on the table?) is not using ASSM and system managed extents the new feature should not be supported.
Here is what I get when I run the same script as above using 23c FREE and an ASSM AUTOALLOCATE tablespace - which should be a supported configuration:
SQL*Plus: Release 21.0.0.0.0 - Production on Wed Aug 16 17:00:29 2023
Version 21.3.0.0.0
Copyright (c) 1982, 2021, Oracle. All rights reserved.
Last Successful login time: Wed Aug 16 2023 16:59:53 +02:00
Connected to:
Oracle Database 23c Free, Release 23.0.0.0.0 - Developer-Release
Version 23.2.0.0.0
SQL> column value_start new_value value_start
SQL> column value_end new_value value_end
SQL> drop table if exists t1 purge;
Table dropped.
SQL> create table t1
2 -- Looks like the restriction on the tablespace isn't implemented properly in 23c FREE!
3 --tablespace TEST_8K
4 tablespace TEST_8K_ASSM_AUTO
5 as
6 select *
7 from all_objects;
Table created.
SQL> select count(*) from t1;
COUNT(*)
----------
75677
SQL> set termout off
SQL> insert /*+ append */ into t1
2 select * from t1;
75677 rows created.
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
1582
SQL> select count(*) from t1;
COUNT(*)
----------
151354
SQL> set termout off
SQL> insert /*+ append */ into t1
2 select * from t1;
151354 rows created.
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
3163
SQL> commit;
Commit complete.
SQL> select count(*) from t1;
COUNT(*)
----------
302708
SQL> set termout off
SQL> insert /*+ append NO_MULTI_APPEND */ into t1
2 select * from t1;
302708 rows created.
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
6325
SQL> select /*+ NO_MULTI_APPEND */ count(*) from t1;
COUNT(*)
----------
302708
SQL> select count(*) from t1;
COUNT(*)
----------
302708
SQL> set termout off
SQL> insert /*+ append */ into t1
2 select * from t1;
insert /*+ append */ into t1
*
ERROR at line 1:
ORA-12838: cannot read/modify an object after modifying it in parallel
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
0
SQL> commit;
Commit complete.
SQL> select count(*) from t1;
COUNT(*)
----------
605416
Some observations: First it's really nice that now in the first part of the script several Direct Path loads are possible within the same transaction, and also accessing the manipulated object using SELECT within the transaction works as expected.
But check closely the second part of the script where I make use of the NO_MULTI_APPEND hint (which is apparently meant by the NO_MULTI_STATEMENT hint mentioned in the official documentation) that should revert to the previous behaviour.
While it is expected now that the second insert in APPEND mode fails again with the known "ORA-12838" error, the SELECT within the same transaction does not raise an error but what is even worse it returns a wrong result - it doesn't count the newly added rows but obviously only sees the rows that existed before the insert was executed.
It becomes even worse if a configuration gets used that isn't supported for the new feature - a tablespace using MSSM and / or uniform extent size - running the same script again only using a different tablespace:
SQL*Plus: Release 21.0.0.0.0 - Production on Wed Aug 16 16:59:53 2023
Version 21.3.0.0.0
Copyright (c) 1982, 2021, Oracle. All rights reserved.
Last Successful login time: Wed Aug 16 2023 16:57:27 +02:00
Connected to:
Oracle Database 23c Free, Release 23.0.0.0.0 - Developer-Release
Version 23.2.0.0.0
SQL> column value_start new_value value_start
SQL> column value_end new_value value_end
SQL>
SQL> drop table if exists t1 purge;
Table dropped.
SQL> create table t1
2 -- Looks like the restriction on the tablespace isn't implemented properly in 23c FREE!
3 tablespace TEST_8K
4 --tablespace TEST_8K_ASSM_AUTO
5 as
6 select *
7 from all_objects;
Table created.
SQL> select count(*) from t1;
COUNT(*)
----------
75677
SQL> set termout off
SQL> insert /*+ append */ into t1
2 select * from t1;
75677 rows created.
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
1580
SQL> select count(*) from t1;
COUNT(*)
----------
75677
SQL> set termout off
SQL> insert /*+ append */ into t1
2 select * from t1;
insert /*+ append */ into t1
*
ERROR at line 1:
ORA-12838: cannot read/modify an object after modifying it in parallel
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
0
SQL> commit;
Commit complete.
SQL> select count(*) from t1;
COUNT(*)
----------
151354
SQL> set termout off
SQL> insert /*+ append NO_MULTI_APPEND */ into t1
2 select * from t1;
151354 rows created.
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
3159
SQL> select /*+ NO_MULTI_APPEND */ count(*) from t1;
COUNT(*)
----------
151354
SQL> select count(*) from t1;
COUNT(*)
----------
151354
SQL> set termout off
SQL> insert /*+ append */ into t1
2 select * from t1;
insert /*+ append */ into t1
*
ERROR at line 1:
ORA-12838: cannot read/modify an object after modifying it in parallel
SQL> set termout off
SQL> set echo off
Diff physical writes direct
---------------------------
0
SQL> commit;
Commit complete.
SQL> select count(*) from t1;
COUNT(*)
----------
302708
Now the same wrong results problem also occurs in the first part of the script - the SELECT within the same transaction doesn't raise an error but shows a wrong result.
Conclusion so far: It's good to see that this very long lasting restriction finally gets lifted in 23c, but at least in 23c FREE you need to be careful - the implementation doesn't seem to be complete yet. Hopefully this will all be sorted out when 23c final gets released.
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