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Don Burleson Blog 







Global temporary table tips

Oracle Database Tips by Donald BurlesonFebruary 22, 2015

Question:  When do I use a global temporary table?  How does a global temporary table work to pass data to subsequent SQL statements?

Answer:  Oracle introduced Global Temporary Tables (GTT) for removing complex subqueries and allowing us to materialize the intermediate data that we need to solve a complex problem with SQL.

A global temporary table is a "template" for a table, that can be populated by any session in Oracle.  Once the session terminates, the global temporary table data disappears.

12c note: Starting in 12c, Oracle will allow you to invoke session-level dbms_stats to gather statistics specific to your own global temporary table. Prior to 12c, statistics were shared from a master copy of the CBO statistics.

A global temporary table can dramatically improve the performance of certain SQL self-join queries that summarize data values.  You can use the global temporary tables (GTT) syntax to improve the speed of queries that perform complex summarization activities.

The Oracle database codified the global temporary table starting in Oracle8i, allowing you to replace complex subqueries with a global temporary table in order to improve the speed and readability of ccomplex queries that must store and pass intermediate data between SQL statements.

See here for the general concept of a global temporary table and a global temporary table example.   Also see my notes on SQL tuning for global temporary tables..

For tuning the underlying temporary tables in SQL, see these important notes on tuning execution plans for global temporary table and WITH clause materializations.


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