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Re: Partition table query performance

From: "Greg Jaman" <gjaman(at)gmail(dot)com>
To: "Gregory Stark" <stark(at)enterprisedb(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Partition table query performance
Date: 2008-11-27 17:25:47
Message-ID: b72893ad0811270925n2ea1a586ked1d3c1fdaf8dba8@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
Thanks Gregory,

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(at)enterprisedb(dot)com>wrote:

> "Greg Jaman" <gjaman(at)gmail(dot)com> writes:
>
> > 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!
>
> I'm afraid this is a known problematic use case of Postgres's current
> 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!
>

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