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Automatic Indexing: Potential Locking Issues Part II (“Don’t Stop”) December 5, 2022

Posted by Richard Foote in 19c, 19c New Features, Automatic Indexing, Autonomous Database, CBO, Exadata, Full Table Scans, Invisible Indexes, Locking Issues, Oracle, Oracle Cloud, Oracle Cost Based Optimizer, Oracle General, Oracle Indexes.
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In my previous post, I highlighted how a long transaction can potentially cause the creation of an Automatic Index to hang due to the inability of the Automatic Indexing process to obtain the necessary locks.

However, these locks can have a much wider consequence, as it’s the entire Automatic Indexing process that is forced to hang, not just the creation of a specific index. This is due to the fact that Automatic Indexing works in a serial fashion, working on one index at a time, in order to put the brakes on the amount of resources that Automatic Indexing can potentially consume.

Therefore, it’s not just the creation of the specifically locked automatic index that is impacted, but the subsequent creation of all Automatic Indexes. No other Automatic Index can be created until the locking issue is resolved.

To highlight, I’m going to create and populate other table:

SQL> create table david_bowie (id number, code number, name varchar2(42));

Table created.

SQL> insert into david_bowie select rownum, mod(rownum, 1000000)+1, 'David Bowie' from dual connect by level <= 10000000;

10000000 rows created.

SQL> commit;

Commit complete.

SQL> exec dbms_stats.gather_table_stats(ownname=>null, tabname=>'DAVID_BOWIE');

PL/SQL procedure successfully completed.

I’ll next run an SQL several times that is forced to perform a Full Table Scan because of a missing index:

SQL> select * from david_bowie where code=42; 10 rows selected. Execution Plan ---------------------------------------------------------- Plan hash value: 1390211489 --------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 10 | 230 | 6714 (2)| 00:00:01 | | * 1 | TABLE ACCESS FULL | DAVID_BOWIE | 10 | 230 | 6714 (2)| 00:00:01 | --------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - storage("CODE"=42) filter("CODE"=42) Statistics ---------------------------------------------------------- 0 recursive calls 0 db block gets 48130 consistent gets 38657 physical reads 0 redo size 885 bytes sent via SQL*Net to client 52 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 10 rows processed However, if we look at the current Automatic Indexing report: SQL> select dbms_auto_index.report_last_activity() report from dual; REPORT -------------------------------------------------------------------------------- GENERAL INFORMATION ------------------------------------------------------------------------------- Activity start : 01-DEC-2022 07:12:31 Activity end : 05-DEC-2022 12:15:42 Executions completed : 0 Executions interrupted : 0 Executions with fatal error : 0 ------------------------------------------------------------------------------- SUMMARY (AUTO INDEXES) ------------------------------------------------------------------------------- Index candidates : 0 Indexes created : 0 Space used : 0 B Indexes dropped : 0 SQL statements verified : 0 SQL statements improved : 0 SQL plan baselines created : 0 Overall improvement factor : 1x ------------------------------------------------------------------------------- SUMMARY (MANUAL INDEXES) ------------------------------------------------------------------------------- Unused indexes : 0 Space used : 0 B Unusable indexes : 0 ------------------------------------------------------------------------------- ERRORS -------------------------------------------------------------------------------- ------------- No errors found. -------------------------------------------------------------------------------- -------------

 

We can see that the Automatic Indexing process is STILL hanging days later from the still uncommitted transaction. Therefore, it’s impossible for an Automatic Index to be created for this new workload, or indeed ANY new workload, until the locking issue is resolved, with the completion of the associated locking transaction.

We can easily see the troublesome lock:

SQL> select * from dba_waiters;

WAITING_SESSION WAITING_CON_ID HOLDING_SESSION HOLDING_CON_ID LOCK_TYPE   MODE_HELD MODE_REQUESTED   LOCK_ID1   LOCK_ID2
--------------- -------------- --------------- -------------- ----------- --------- -------------- ---------- ----------
            164              3             167              3 Transaction Exclusive Share              327694      10623

 

As a consequence, no new Automatic Index can be created for this new workload:

SQL> select index_name, auto, constraint_index, visibility, status, num_rows, leaf_blocks
from user_indexes where table_name='DAVID_BOWIE';

no rows selected

And the existing workload remains inefficient:

SQL> select * from david_bowie where code=42;

10 rows selected.

Execution Plan
----------------------------------------------------------
Plan hash value: 1390211489

---------------------------------------------------------------------------------
|  Id | Operation          | Name        | Rows | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |             |   10 |   230 |    6714 (2)| 00:00:01 |
| * 1 |  TABLE ACCESS FULL | DAVID_BOWIE |   10 |   230 |    6714 (2)| 00:00:01 |
---------------------------------------------------------------------------------

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

    1 - storage("CODE"=42)
        filter("CODE"=42)

Statistics
----------------------------------------------------------
          0 recursive calls
          0 db block gets
      48130 consistent gets
      38657 physical reads
          0 redo size
        885 bytes sent via SQL*Net to client
         52 bytes received via SQL*Net from client
          2 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
         10 rows processed

 

Once the locking transaction is finally completed:

SQL> insert into bowie_busy values (10000001, 42, 'Ziggy Stardust');

1 row created.

SQL> commit;

Commit complete.

The Automatic Indexing process can again resume and the new Automatic Indexes can finally be created as necessary:

SQL> select dbms_auto_index.report_last_activity() report from dual;

REPORT
--------------------------------------------------------------------------------
GENERAL INFORMATION
-------------------------------------------------------------------------------
Activity start              : 05-DEC-2022 12:30:30
Activity end                : 05-DEC-2022 12:31:22
Executions completed        : 1
Executions interrupted      : 0
Executions with fatal error : 0
-------------------------------------------------------------------------------

SUMMARY (AUTO INDEXES)
-------------------------------------------------------------------------------
Index candidates                      : 0
Indexes created (visible / invisible) : 2 (0 / 2)
Space used (visible / invisible)      : 287.31 MB (0 B / 287.31 MB)
Indexes dropped                       : 0
SQL statements verified               : 3
SQL statements improved               : 0
SQL plan baselines created            : 0
Overall improvement factor            : 1x
-------------------------------------------------------------------------------

SUMMARY (MANUAL INDEXES)
-------------------------------------------------------------------------------
Unused indexes   : 0
Space used       : 0 B
Unusable indexes : 0
-------------------------------------------------------------------------------

INDEX DETAILS
-------------------------------------------------------------------------------
1. The following indexes were created:
-------------------------------------------------------------------------------
---------------------------------------------------------------------------
| Owner | Table       | Index                | Key  | Type   | Properties |
---------------------------------------------------------------------------
| BOWIE | BOWIE_BUSY  | SYS_AI_8pkdh6q096qvs | CODE | B-TREE | NONE       |
| BOWIE | DAVID_BOWIE | SYS_AI_czmkjhqr21732 | CODE | B-TREE | NONE       |
---------------------------------------------------------------------------
-------------------------------------------------------------------------------

ERRORS
--------------------------------------------------------------------------------
-------------
No errors found.
--------------------------------------------------------------------------------
-------------

 

If you find that the Automatic Indexing process has hung, check to make sure there are no long locks on associated underlying tables that could be causing the whole Automatic Index process to freeze…

 

NOTE: This post is dedicated to the memory of Christine McVie, who recently passed away…

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