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(at)peak6(dot)com

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