The following bug has been logged online:

Bug reference: 2208
Logged by: Fahri CAKIROGLU
Email address: fahri (AT) gbks (DOT) com.tr
PostgreSQL version: 8.1
Operating system: windows nt 2000
Description: Low performance on select
Details:

Consecutive selects from three different tables
causes low performance(about 1500 ms).
Same select count from two tables gives good performance(about 30 ms).
All searched keys are indexed and individual execution
of each query is very fast.

Slow query :
select * from orders where ordernum='1' order by ordernum limit 1;
select * from customers where custnum='1' order by custnum limit 1;
select * from invoice where invnum='1' order by inv limit 1;

Query with good performance :
select * from orders where ordernum='1' order by ordernum limit 1;
select * from customers where custnum='1' order by custnum limit 1;
select * from orders where ordernum='2' order by ordernum limit 1;
select * from customers where custnum='2' order by custnum limit 1;

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On Wed, Jan 25, 2006 at 12:32:31PM +0000, Fahri CAKIROGLU wrote:
This might be due to caching. When you query two tables all of the
pages you need might be cached, but when you query a third table
some of the pages from the other tables might be evicted from the
cache. When you query one of those tables again the pages have to
be fetched from disk.

How big are the tables in question?

Could you post the EXPLAIN ANALYZE output of each query? Have you
tuned any of your postgresql.conf settings, in particular shared_buffers?
How much memory do you have?

--
Michael Fuhr

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