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HASH (UNIQUE)

Description

Provides a more efficient implementation of the sort algorithm for DISTINCT statements

Versions

This operation is implemented in the following versions

This operation replaces the SORT(UNIQUE) operation in limited circumstances

Example

This example was developed using Oracle 10.2.0.1 on Linux

This example requires the following table definitions

    CREATE TABLE t1 (c1 NUMBER);

The statement

    SELECT DISTINCT (c1) FROM t1;

generates the following execution plan

0     SELECT STATEMENT Optimizer=CHOOSE
1   0   HASH (UNIQUE)
2   1     TABLE ACCESS (FULL) OF 'T1'

If the "_gby_hash_aggregation_enabled" parameter is set to FALSE (default TRUE) then the HASH (UNIQUE) operation is disabled. For example

    ALTER SESSION SET "_gby_hash_aggregation_enabled" = FALSE;

Now the statement

    SELECT DISTINCT (c1) FROM t1;

generates the following execution plan

0     SELECT STATEMENT Optimizer=CHOOSE
1   0   SORT (UNIQUE)
2   1     TABLE ACCESS (FULL) OF 'T1'