Re: Planner statistics vs. count(*)

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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