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Re: partitioned table and ORDER BY indexed_field DESC LIMIT 1

From: Mark Kirkwood <markir(at)paradise(dot)net(dot)nz>
To: Luke Lonergan <llonergan(at)greenplum(dot)com>
Cc: Gregory Stark <stark(at)enterprisedb(dot)com>, Heikki Linnakangas <heikki(at)enterprisedb(dot)com>, Anton <anton200(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: partitioned table and ORDER BY indexed_field DESC LIMIT 1
Date: 2007-11-08 07:18:47
Message-ID: 4732B857.6070102@paradise.net.nz (view raw or flat)
Thread:
Lists: pgsql-performance
Luke Lonergan wrote:
> On 11/7/07 10:21 PM, "Gregory Stark" <stark(at)enterprisedb(dot)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

In response to

pgsql-performance by date

Next:From: Guillaume SmetDate: 2007-11-08 10:08:33
Subject: Re: Estimation problem with a LIKE clause containing a /
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Subject: Re: partitioned table and ORDER BY indexed_field DESC LIMIT 1

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