From: Mark Kirkwood <markir@paradise.net.nz>
To: Luke Lonergan <llonergan@greenplum.com>
Cc: Gregory Stark <stark@enterprisedb.com>
Cc: Heikki Linnakangas <heikki@enterprisedb.com>
Cc: Anton <anton200@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: partitioned table and ORDER BY indexed_field DESC LIMIT 1
Date: Thu, 08 Nov 2007 20:18:47 +1300
Message-ID: <4732B857.6070102@paradise.net.nz> (raw)
In-Reply-To: <C357EF54.44FA2%llonergan@greenplum.com>
References: <C357EF54.44FA2%llonergan@greenplum.com>
Luke Lonergan wrote:
> On 11/7/07 10:21 PM, "Gregory Stark" <stark@enterprisedb.com> wrote:
>
>
>>> part=# explain SELECT * FROM n_traf ORDER BY date_time LIMIT 1;
>>> QUERY PLAN
>>> -----------------------------------------------------------------------------
>>> --------------------------------------------------------------------
>>> Limit (cost=198367.14..198367.15 rows=1 width=20)
>>> -> Sort (cost=198367.14..200870.92 rows=1001510 width=20)
>>> Sort Key: public.n_traf.date_time
>>> -> Result (cost=0.00..57464.92 rows=1001510 width=20)
>>> -> Append (cost=0.00..57464.92 rows=1001510 width=20)
>>> -> Index Scan using n_traf_date_time_login_id on n_traf
>>> (cost=0.00..66.90 rows=1510 width=20)
>>>
>> That looks suspicious. There's likely no good reason to be using the index
>> scan unless it avoids the sort node above the Append node. That's what I hope
>> to do by having the Append executor code do what's necessary to maintain the
>> order.
>>
>
> Yah - the way it works in GPDB is that you get a non-sorting plan with an
> index scan below the parent - that was the point of the fix. Hmm.
>
>
Unfortunately our plan in GPDB looks exactly the same in this case - so
we have a bit of work to do as well! Initially I wondered if I have got
something wrong in the patch... and checked on GPDB - only to see the
same behaviour! (see prev comment about LIMIT).
Cheers
Mark
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To: pgsql-performance@postgresql.org
Cc: markir@paradise.net.nz, llonergan@greenplum.com, stark@enterprisedb.com, heikki@enterprisedb.com, anton200@gmail.com
Subject: Re: partitioned table and ORDER BY indexed_field DESC LIMIT 1
In-Reply-To: <4732B857.6070102@paradise.net.nz>
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