From: Florian Weimer <fweimer@bfk.de>
To: pgsql-performance@postgresql.org
Subject: Odd estimation issue with user-defined type
Date: Fri, 03 Sep 2010 14:15:19 +0000
Message-ID: <82zkvyncaw.fsf@mid.bfk.de> (raw)
I've created a few user-defined types quite similar to uuid which we
use to store various hashes in the database. (The types use binary
encoding internally, but only expose hexadecimal strings externally.)
The hashes are roughly equidistributed, so when I do a range query
which is essentially based on a hash prefix(*), I expect the result to
contain N * 2**(-k) results, where N is the table size and k the
number of bits in the range. Actual query results show that this is
the case. The odd thing is that the planner thinks that the range
query will return about one quarter of the table, independently of the
range specified. Of course, the row estimates are quite far off as a
result, leading to suboptimal plans.
Any idea what could cause this? Do I need to provide some estimator
function somewhere?
(*) I don't use LIKE, because its optimization is hard-coded to a few
types, but explicit BETWEEN ... AND queries.
--
Florian Weimer <fweimer@bfk.de>
BFK edv-consulting GmbH http://www.bfk.de/
Kriegsstraße 100 tel: +49-721-96201-1
D-76133 Karlsruhe fax: +49-721-96201-99
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: fweimer@bfk.de
Subject: Re: Odd estimation issue with user-defined type
In-Reply-To: <82zkvyncaw.fsf@mid.bfk.de>
* 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