I was on IRC yesterday and a few people indicated the same thing...
Searching for the last reading is a very important function for our database. I wrote the below function searches all child tables for the max. It is not optimization because it doesn't omit tables by look at the check constraints on child tables to see if the last found max is greater than the constraints. Right now this function executes in 50ms vs the 80+ for the same query against the partition set.
create or replace function Data_max(in_sensorID integer) returns bigint AS
$$
DECLARE
childtable RECORD;
childres RECORD;
max_dataID bigint := NULL;
max_ts timestamp without time zone;
BEGIN
FOR childtable in select pc.relname as relname from pg_class pc join pg_inherits pi on pc.oid=pi.inhrelid where inhparent=(select oid from pg_class where relname='Data')
LOOP
EXECUTE ' SELECT "dataID", ts FROM ' || quote_ident( childtable.relname )
|| ' WHERE "sensorID"=' || quote_literal(in_sensorID) || ' order by ts desc limit 1 ' INTO childres;
IF childres is not NULL THEN
IF max_ts is NULL OR childres.ts > max_ts THEN
max_ts:= childres.ts;
max_dataID:= childres."dataID";
END IF;
END IF;
END LOOP;
return max_dataID;
END;
$$
language 'plpgsql';
On Wed, Nov 26, 2008 at 4:48 PM, Gregory Stark <stark@xxxxxxxxxxxxxxxx> wrote:
"Greg Jaman" <gjaman@xxxxxxxxx> writes:I'm afraid this is a known problematic use case of Postgres's current
> I have a problem with partitioning and I'm wondering if anyone can provide
> some insight. I'm trying to find the max value of a column across multiple
> partitions. The query against the partition set is quite slow while queries
> against child partitions is very fast!
partitioning support. Postgres is not capable of finding the plan which you're
undoubtedly looking for where it uses the same plan as your child table query
iterating over the partitions.
There are several groups working to improve this in different ways but none of
them appear to be on track to be in 8.4 so it will be 8.5 or later before they
appear. Sorry.
--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's 24x7 Postgres support!
