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Re: seq scan in the case of max() on the primary key column

From: Shaun Thomas <sthomas(at)peak6(dot)com>
To: Magnus Hagander <magnus(at)hagander(dot)net>
Cc: Svetlin Manavski <svetlin(dot)manavski(at)gmail(dot)com>,<pgsql-performance(at)postgresql(dot)org>
Subject: Re: seq scan in the case of max() on the primary key column
Date: 2011-06-16 18:36:31
Message-ID: 4DFA4D2F.8020101@peak6.com (view raw or flat)
Thread:
Lists: pgsql-performance
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(at)peak6(dot)com

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