Reply-To: <alexandra@trymedia.com>
From: "Alexandra Birch" <alexandra@trymedia.com>
To: <bruno@wolff.to>
Cc: <pgsql-sql@postgresql.org>
Subject: Re: limit 1 and functional indexes
Date: Thu, 29 Jan 2004 16:02:06 +0100
Message-ID: <BJELKOAELOIHMLJIEGHJOEAAEOAA.alexandra@trymedia.com>
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> >
> > Postgres choses the wrong index when I add limit 1 to the query.
> > This should not affect the index chosen.
>
> I don't know the complete answer to your question, but since no one else
> has commented I will answer what I can.

Thanks - your reply is apreciated :)

> It IS reasobable for the planner to choose a different plan when you
> add a LIMIT clause to a query.

OK - I'll investigate this further.

> > I read that functional indexes are sometimes not chosen correctly by
> > optimizer.
>
> I don't believe there are any particular problems with functional indexes.
> The opitmizer isn't perfect and will sometimes choose poor plans.

OK - but there was some discussion about statistics for functional indexes, for eg:
http://archives.postgresql.org/pgsql-general/2004-01/msg00978.php
This does not help me solve my problem though :)

> > Is there anything I can do to always use the functional index in the
> > following queries?
>
> Have you done an ANALYZE of the table recently?

Yip - I should have said we do a daily VACUUM ANALYZE.

> It might be useful to see the EXPLAIN ANALYZE output, rather than just
> the EXPLAIN output, as that will give the actual times needed to do
> the various steps.

I thought the cost values would be enough from the EXPLAIN alone.
And the query takes so long to run :(

Here is the output of EXPLAIN ANALYZE first with limit 1 then without:

explain analyze
select code
from transactions
where UPPER(pop) = UPPER('79bcdc8a4a4f99e7c111111111111111')
order by order_date DESC LIMIT 1;
--------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..332.44 rows=1 width=33) (actual time=377745.75..377745.75 rows=0 loops=1)
   ->  Index Scan Backward using transactions_date_aff on transactions  (cost=0.00..982549.96 rows=2956 width=33) (actual
time=377718.61..377718.61 rows=0 loops=1)
         Filter: (upper((pop)::text) = '79BCDC8A4A4F99E7C111111111111111'::text)
 Total runtime: 378439.32 msec

explain analyze
select code
from transactions
where UPPER(pop) = UPPER('79bcdc8a4a4f99e7c111111111111111')
order by order_date DESC;
                                                                   QUERY PLAN
------------------------------------------------------------------------------------------------------------------------------------
-------------
 Sort  (cost=11824.16..11831.55 rows=2956 width=33) (actual time=248.17..248.17 rows=0 loops=1)
   Sort Key: order_date
   ->  Index Scan using transactions_pop_i on transactions  (cost=0.00..11653.79 rows=2956 width=33) (actual time=126.13..126.13
rows=0 loops=1)
         Index Cond: (upper((pop)::text) = '79BCDC8A4A4F99E7C111111111111111'::text)
 Total runtime: 248.25 msec

Thank you,

Alexandra