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

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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