From: Richard Huxton <dev@archonet.com>
To: Gaetano Mendola <mendola@bigfoot.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Wrong plan sequential scan instead of an index one
Date: Fri, 30 Mar 2007 11:33:25 +0100
Message-ID: <460CE775.8080908@archonet.com> (raw)
In-Reply-To: <460CE359.8060208@bigfoot.com>
References: <460CD941.6080303@bigfoot.com>
<460CDCDC.7020901@archonet.com>
<460CE359.8060208@bigfoot.com>
Gaetano Mendola wrote:
>
> Richard Huxton wrote:
>>
>> Now, why 19 rows from the subquery should produce such a large estimate
>> in the outer query I'm not sure. Any strange distribution of values on
>> pvcp?
>
> I don't know what do you mean for strange, this is the distribution:
>
> test=# select count(*) from t_oa_2_00_card;
> count
> - --------
> 877682
> (1 row)
>
> test=# select count(*), pvcp from t_oa_2_00_card group by pvcp;
> count | pvcp
> - -------+------
> (92 rows)
>
>
> I think that estimate is something like: 877682 / 92 * 19
So if you actually had 19 matches for '%pi%' it might be a sensible plan
then. I'm afraid I don't know of any way to improve PG's prediction on
how many matches you'll get for a substring pattern though.
--
Richard Huxton
Archonet Ltd
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-performance@postgresql.org
Cc: dev@archonet.com, mendola@bigfoot.com
Subject: Re: Wrong plan sequential scan instead of an index one
In-Reply-To: <460CE775.8080908@archonet.com>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox