From: | Bricklen Anderson <BAnderson(at)PresiNET(dot)com> |
---|---|
To: | evgeny gridasov <eugrid(at)fpm(dot)kubsu(dot)ru> |
Cc: | pgsql-performance(at)postgresql(dot)org |
Subject: | Re: Planner statistics vs. count(*) |
Date: | 2005-09-20 18:29:31 |
Message-ID: | 4330550B.80600@PresiNET.com |
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Thread: | |
Lists: | pgsql-performance |
evgeny gridasov wrote:
> Hi Everybody.
>
> I am going to replace some 'select count(*) from ... where ...' queries
> which run on large tables (10M+ rows) with something like
> 'explain select * from ... where ....' and parse planner output after that
> to find out its forecast about number of rows the query is going to retrieve.
>
> Since my users do not need exact row count for large tables, this will
> boost performance for my application. I ran some queries with explain and
> explain analyze then. If i set statistics number for the table about 200-300
> the planner forecast seems to be working very fine.
>
> My questions are:
> 1. Is there a way to interact with postgresql planner, other than 'explain ...'? An aggregate query like 'select estimate_count(*) from ...' would really help =))
> 2. How precise is the planner row count forecast given for a complex query (select with 3-5 joint tables,aggregates,subselects, etc...)?
>
>
I think that this has been done before. Check the list archives (I believe it
may have been Michael Fuhr?)
ah, check this:
http://archives.postgresql.org/pgsql-sql/2005-08/msg00046.php
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