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Don Burleson Blog 







global_temp_table_stats tips

Oracle Database Tips by Donald BurlesonMay 24, 2016

Question: What does the new 12c global_temp_table_stats do?  When would I want to use global_temp_table_stats on a global temporary table?

Answer:  Prior to 12c, Oracle global temporary tables (GTT) shared a common set of CBO statistics.  New in 12c we see the global_temp_table_stats procedure that allows coders to collect optimizer statistics on individual GTT's.

In most databases the GTT does not have significantly different characteristics to warrant separate statistics, but for those rare cases when the GTT is significantly different from the "norm", the ability to analyze statistics will improve the execution plans of the SQL that reads the GTT data.

The values can be either SESSION or SHARED



Here we set the global_temp_table_stats to SHARED, to allow for individual CBO statistics on specific GTT's:

   DBMS_STATS.set_global_prefs (
    pvalue => 'SHARED');

Rampant author Dr. Tim Hall notes that when session-private statistics are enabled, the following behavior is present.

  • Statistics gathered on a GTT are only available to the optimizer in the current session.
  • If session-private statistics are present, they will be used in preference to the shared statistics.
  • Gathering statistics will invalidate any related cursors in the current session only.
  • Statistics are deleted as soon as the session ends.
  • Pending statistics are not supported for GTTs.

As we see the ability to collect CBO statistics on a individual GTT is rarely needed (to improve SQL execution plans) but this new 12c feature is very useful in those cases where the presence of GTT statistics will influence an execution plan.

There is an "upgrade gotcha" you need to be aware of. If you have upgraded to Oracle Database 12c and are using optimizer_features_enable set to a pre- version (let's say, for the sake of argument, it is '') then be aware that the Oracle Optimizer will not "see" session private statistics.

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