Optimizing Update with WHERE x IN (id1,id2,...)

From: Michael Monnerie <michael(dot)monnerie(at)is(dot)it-management(dot)at>
To: pgsql-admin(at)postgresql(dot)org
Subject: Optimizing Update with WHERE x IN (id1,id2,...)
Date: 2010-12-22 09:59:37
Message-ID: 201012221059.38145@zmi.at
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Dear devs,

I just saw this EXPLAIN in PostgreSQL 8.3.12:

EXPLAIN UPDATE dbmail_messages SET recent_flag = 0 WHERE message_idnr IN
(3506054,3506052,3506050,3506048,3506046,3506044,3506042,3506040,3506038,3506036,3506034,3506032,3506030)
AND recent_flag = 1;
QUERY
PLAN
------------------------------------------------------------------------------------------------------------------------------------------------------------------
Bitmap Heap Scan on dbmail_messages (cost=55.39..105.04 rows=1
width=75)
Recheck Cond: (message_idnr = ANY
('{3506054,3506052,3506050,3506048,3506046,3506044,3506042,3506040,3506038,3506036,3506034,3506032,3506030}'::bigint[]))
Filter: (recent_flag = 1)
-> Bitmap Index Scan on dbmail_messages_pkey (cost=0.00..55.39
rows=13 width=0)
Index Cond: (message_idnr = ANY
('{3506054,3506052,3506050,3506048,3506046,3506044,3506042,3506040,3506038,3506036,3506034,3506032,3506030}'::bigint[]))
(5 rows)

And it seems to me that could be optimized to skip the "Recheck Cond" as
that is already checked by the Index Scan? Should save some CPU cycles.

--
mit freundlichen Grüssen,
Michael Monnerie, Ing. BSc

it-management Internet Services: Protéger
http://proteger.at [gesprochen: Prot-e-schee]
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