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Oracle hash_area_size

Oracle Tips by Burleson
Don Burleson

Question:  How does the hash_area_size parameter work?  Can I use the hash_area_size if I am using AMM (memory_max)?

Answer:  Oracle hash_area_size is simple. The hash_area_size parameter value defaults to 1.5 times sort_area_size and is used for performing hash joins of Oracle tables. The higher the value for hash_area_size, the higher the propensity for the CBO to use a hash join.

The values for hash_area_size is quickly shown with the Oracle show parameters command:

SQL> show parameters hash_area_size


hash_area_size integer 1048576

The hash_area_size is obsolete if you are using pga_aggregate_target, but in Oracle with pga_aggregate_target, a hash area size cannot exceed 5% of the PGA area.

Many increase the hash_area_size with "alter session set hash_area_size" or with a use_hash hint.

Increasing Hash Joins with hash_area_size

To force hash joins you must perform two steps.  It may not be enough to increase the hash_area_size if the CBO is stubborn, and usually you must force the hash join with a hint.

Step 1 - Increase the hash_area_size maximum

alter session set workarea_size_policy=manual;
alter session set hash_area_size=1048576000;

Step 2 - Add a use_hash hint to the SQL

select /*+ use_hash(a, b)*/
from . . .

Scripts for measuring changes to hash_area_size can be found in our script collection:



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