Striving for Optimal Performance

As you can read in the documentation, the columns MAX_TEMPSEG_SIZE and LAST_TEMPSEG_SIZE in the dynamic performance view V$SQL_WORKAREA provide information about the size of the temporary segment used for a specific workarea. The values are given in bytes. Let’s perform a test to check this information…

• Create a test table that contains about 1MB of data:

SQL> CREATE TABLE t AS
  2  SELECT rownum AS id, dbms_random.string('p',1000) AS pad
  3  FROM dual
  4  CONNECT BY level <= 1000;

SQL> execute dbms_stats.gather_table_stats(user, 't')

• Setup the session to force the user process to spill into a temporary segment:

SQL> ALTER SESSION SET workarea_size_policy = manual;
SQL> ALTER SESSION SET sort_area_size = 524288;

• Run test query including a sort operation (that spills to the temporary tablespace):

SQL> SELECT id FROM t ORDER BY pad;

• Check the amount of used temporary space by querying V$SQL_WORKAREA:

SQL> SELECT max_tempseg_size, last_tempseg_size
  2  FROM v$sql_workarea
  3  WHERE (sql_id, child_number) IN (SELECT prev_sql_id, prev_child_number
  4                                   FROM v$session
  5                                   WHERE sid = sys_context('userenv','sid'));

MAX_TEMPSEG_SIZE LAST_TEMPSEG_SIZE
---------------- -----------------
         2097152           2097152

According to this information the size of the temporary space used to execute the query was 2MB. So far, so good.

Always according to the documentation another dynamic performance view, V$SQL_PLAN_STATISTICS_ALL, should provide the same information (remember, V$SQL_PLAN_STATISTICS_ALL shows in a single view all the information provided by the views V$SQL_PLAN, V$SQL_PLAN_STATISTICS, and V$SQL_WORKAREA). Let’s check it…

• Run the same test query as before:

SQL> SELECT id FROM t ORDER BY pad;

• Check the amount of used memory by querying V$SQL_PLAN_STATISTICS_ALL:

SQL> SELECT max_tempseg_size, last_tempseg_size
  2  FROM v$sql_plan_statistics_all
  3  WHERE (sql_id, child_number) IN (SELECT prev_sql_id, prev_child_number
  4                                   FROM v$session
  5                                   WHERE sid = sys_context('userenv','sid'))
  6  AND max_tempseg_size IS NOT NULL;

MAX_TEMPSEG_SIZE LAST_TEMPSEG_SIZE
---------------- -----------------
            2048              2048

Ups! According to this information the size of the temporary space used to execute the query was 2KB. Mhmm, something is not good… For this reason, at the end of 2007 I opened a service request about this issue. The support guy recognized the problem and opened a bug. Fine. For some unknown reasons (?) yesterday I was checking the status of few bugs. While doing so I noticed that this specific bug was closed few months ago with the status “Could Not Reproduce”! I don’t know you, but on my 64-bit Linux server I can reproduce it with at least 11.1.0.7.0, 11.1.0.6.0, 10.2.0.4.0, 10.2.0.3.0, 10.2.0.2.0, 10.2.0.1.0, 10.1.0.5.0, 10.1.0.4.0 and 10.1.0.3.0. Geez!

It is essential to note that also the package DBMS_XPLAN shows wrong information (here an example for the same query as before):

SQL> SELECT * FROM table(dbms_xplan.display_cursor(null,null,'memstats last'));

PLAN_TABLE_OUTPUT
---------------------------------------------------------------------------------
SQL_ID  ftb71b6926dtn, child number 0
-------------------------------------
SELECT id FROM t ORDER BY pad

Plan hash value: 961378228

---------------------------------------------------------------------------------
| Id  | Operation          | Name | E-Rows |  OMem |  1Mem | Used-Mem | Used-Tmp|
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |        |       |       |          |         |
|   1 |  SORT ORDER BY     |      |   1000 |  1152K|   562K|  529K (1)|    2048 |
|   2 |   TABLE ACCESS FULL| T    |   1000 |       |       |          |         |
---------------------------------------------------------------------------------

The only good thing about the fact that Oracle is not willing to fix the bug is that my book, Troubleshooting Oracle Performance, does not need to be updated. In fact, at page 210, while describing the output of the package DBMS_XPLAN I wrote the following information:

• Used-Tmp: The amount of temporary space used by the operation during the last execution. This value must be multiplied by 1,024 to be consistent with the other memory utilization columns (for example, 32K means 32MB).
• Max-Tmp: The maximum amount of temporary space used by the operation. This value has to be multiplied by 1,024 to be consistent with the other memory utilization columns (for example, 32K means 32MB).

ADDENDA (Mai 6, 2009): This post was noticed by an Oracle employee and, as a result, the bug was reopened. Thank you Greg!