From: Shaun Thomas <sthomas@peak6.com>
To: Magnus Hagander <magnus@hagander.net>
Cc: Svetlin Manavski <svetlin.manavski@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: seq scan in the case of max() on the primary key column
Date: Thu, 16 Jun 2011 13:36:31 -0500
Message-ID: <4DFA4D2F.8020101@peak6.com> (raw)
In-Reply-To: <BANLkTikuFEyOse=sUoH0BXYrpUW=7Pp_tw@mail.gmail.com>
References: <BANLkTikntdSrhneCPzPutY9T-sAQpFXTMw@mail.gmail.com>
<BANLkTikuFEyOse=sUoH0BXYrpUW=7Pp_tw@mail.gmail.com>
On 06/16/2011 12:25 PM, Magnus Hagander wrote:
> PostgreSQL 9.0 is unable to use an index scan to find min/max on a
> partitioned table. 9.1, however, can do that.
Unfortunately this is true. You can fake it this way though:
/**
* Return the Maximum INT Value for a Partitioned Table Column
*
* @param string Name of Schema of the base partition table.
* @param string Name of the base partition table.
* @param string Name of column to search.
*/
CREATE OR REPLACE FUNCTION spc_max_part_int(VARCHAR, VARCHAR, VARCHAR)
RETURNS INT AS
$$
DECLARE
sSchema ALIAS FOR $1;
sTable ALIAS FOR $2;
sColName ALIAS FOR $3;
sChild VARCHAR;
nMax INT;
nTemp INT;
nParent OID;
BEGIN
EXECUTE '
SELECT max(' || sColName ||')
FROM ONLY ' || sSchema || '.' || quote_ident(sTable)
INTO nMax;
SELECT INTO nParent t.oid
FROM pg_class t
JOIN pg_namespace n ON (t.relnamespace=n.oid)
WHERE n.nspname = sSchema
AND t.relname = sTable;
FOR sChild IN
SELECT t.relname
FROM pg_class t
JOIN pg_inherits c ON (c.inhrelid=t.oid AND c.inhparent=nParent)
LOOP
nTemp := utility.spc_max_part_int(sSchema, sChild, sColName);
nMax := greatest(nTemp, nMax);
END LOOP;
RETURN nMax;
END;
$$ LANGUAGE plpgsql STABLE;
You can call that instead of max, and it'll be much faster. You can
create an analog for min if you need it. So for this, you'd call:
SELECT spc_max_part_int('appqosdata', 'tcpsessions', 'id');
Someone probably has a better solution. :)
--
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 800 | Chicago IL, 60604
312-676-8870
sthomas@peak6.com
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To: pgsql-performance@postgresql.org
Cc: sthomas@peak6.com, magnus@hagander.net, svetlin.manavski@gmail.com
Subject: Re: seq scan in the case of max() on the primary key column
In-Reply-To: <4DFA4D2F.8020101@peak6.com>
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