Re: _gby_hash_aggregation_enabled=false



I've just tried the following on 10.2.0.1
   select owner, count(*) from dba_objects
   group by owner;

Autotrace produced a plan with a hash group by.

Then I did
alter session set "_gby_hash_aggregation_enabled" = false;
and repeated the query - the plan switched to a sort group by

(Your sample query from v$sql_plan defaulted to a sort
group by anyway, which is why I had to find another query).


A possible explanation for your observation - if I ran the test
using SQL_TRACE rather than autotrace, the first cursor
was not invalidated when I changed the setting for the parameter. Try repeating your test with a comment in the text to make the before and after versions of the query 
look different.

You might also try the /*+ no_use_hash_aggregation */ hint.



Regards

Jonathan Lewis
http://jonathanlewis.wordpress.com

Author: Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html

The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html
----- Original Message ----- 
Subject: _gby_hash_aggregation_enabled=false
Date: Thu, 29 Nov 2007 15:10:03 -0700
From: "Allen, Brandon" <Brandon.Allen@xxxxxxxxxxx>

Has anyone else tried setting this hidden parameter and found it didn't
disable the HASH GROUP BY feature?  It's suggested in Metalink 387958.1
as a workaround to the "wrong results" bug (4604970) with the new HASH
GROUP BY feature, so I just set it as follows:
SQL> alter system set "_gby_hash_aggregation_enabled"=false;

System altered.


Then I ran a 10046 trace on a query known to have the HASH GROUP BY
operation in its explain plan:

SQL> alter session set sql_trace=true;



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