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Oracle PGA sort hash _smm_max_size

Oracle Tips by Burleson Consulting
February 6, 2004

When Oracle replaced the sort_area_size and hash_area_size functionality with the  pga_aggregate_target parameter in Oracle, there was a great deal of speculation about how to control sorting and hash joins within the large PGA region.  Here we discuss two issues.  How do we increase the RAM for hash joins and RAM sorting within the pga_aggregate_target?

For example, we might have a single evening batch task that would benefit from using all of the pga_aggregate_target, not just the 5% limit.

Increasing Hash Joins

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 . . .


Increasing RAM sorting

In Oracle, the sorting default is that no single task may consume more than 5% of the pga_aggregate_target region before the sort pages-out to the TEMP tablespace for a disk sort.  For parallel sorts, the limit is 30% of the PGA aggregate, regardless of the number of parallel processes.

You can override the default sorting behavior in two ways:

Option 1 - Manual override:

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

Option 2 - Bounce with special hidden parameter:  In this example, we increase the default amount of RAM available to sort operations from 5% to about 50%:

pga_aggregate_target=10g
_smm_max_size=4000000;

Not that the number for _smm_max_size is expressed in k-bytes, so this value is about 4.5 gigabytes.

Remember, hidden parameters are totally unsupported, so use this technique at your own risk.


If you like Oracle tuning, see my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts:

http://www.rampant-books.com/book_1002_oracle_tuning_definitive_reference_2nd_ed.htm 

 


 

 

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