Message-ID: <49236288.3000006@fuzzy.cz>
Date: Wed, 19 Nov 2008 01:49:12 +0100
From: Tomas Vondra <tv@fuzzy.cz>
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CC: pgsql-performance@postgresql.org
Subject: Re: Performance and IN clauses
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> I know that it's much faster (for us) to run many smaller queries than
> one large query, and I think that it's primarily because of your reason
> a.  Most of our problems come from Pg misunderstanding the results of a
> join and making a bad plan decision.  Batching dramatically reduces the
> liklihood of this.
> 
> -Mark

Show us the plan (output of EXPLAIN ANALYZE), along with detailed info 
about the table (number of rows / pages) and environment (amount of RAM, 
etc.). Without these information it's impossible to tell which of the 
choices is right.

In my experience the planner is a very good piece of software, but if 
you feed him with bad information about the environment / data in the 
beginning, you can hardly expect good plans.

Occasionally I provide consultancy to developers having problems with 
PostgreSQL, and while most of the time (say 70% of the time) the 
problems are caused by mistakes in SQL or incorrect design of the 
system, problems with proper settings of the PostgreSQL are quite often. 
I really don't know if you use the default values or if you have tuned 
the settings (and how), but it's hard to tell from your original post.

For example if you don't set the work_mem according to your settings, 
this may result in on-disk sorting even if you have plenty of free 
memory. Or if you have fast drives, the default cost settings may 
produce bad plans (index scan may seem too expensive), etc. And of 
course you may have data with complicated statistical properties, and 
the default level of details may not be sufficient (try increase it with 
SET STATISTICS for the column).

Anyway - there may be glitch / corner case in the planner of course, but 
   it's hard to tell without the EXPLAIN ANALYZE output.

regards
Tomas