Message-ID: <1140018967.43f34f1761803@imp2-g19.free.fr>
Date: Wed, 15 Feb 2006 16:56:07 +0100
From: martial.bizel@free.fr
To: pgsql-performance@postgresql.org
Subject: Re: explain hashAggregate
References: <20060215090446.2F3AC1B819F@mx1.hub.org>
	<1140018387.43f34cd36a076@imp2-g19.free.fr>
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Good morning,

I try to understand how optimizer uses HashAggregate instead of GroupAggregate
and I want to know what is exactly this two functionnality (benefits
/inconvenients)

In my case, I've this explain plan.
-----------------------
 Nested Loop  (cost=2451676.23..2454714.73 rows=1001 width=34)
   ->  Subquery Scan "day"  (cost=2451676.23..2451688.73 rows=1000 width=16)
         ->  Limit  (cost=2451676.23..2451678.73 rows=1000 width=12)
               ->  Sort  (cost=2451676.23..2451684.63 rows=3357 width=12)
                     Sort Key: sum(occurence)
                     ->  HashAggregate  (cost=2451471.24..2451479.63 rows=3357
width=12)
                           ->  Index Scan using test_date on
queries_detail_statistics  (cost=0.00..2449570.55 rows=380138 width=12)
                                 Index Cond: ((date >= '2006-01-01'::date) AND
(date <= '2006-01-30'::date))
                                 Filter: (((portal)::text = '1'::text) OR
((portal)::text = '2'::text))
   ->  Index Scan using query_string_pkey on query_string  (cost=0.00..3.01
rows=1 width=34)
         Index Cond: ("outer".query = query_string.id)
----------------------------
How to get necessary memory RAM for this explain plan ?


thanks a lot