Queries take forever on ported database from MSSQL -> Postgresql

Hi all,

I've spent the last two weeks trying to solve this problem, reading all
the documentation I can lay my hands on, and searching the archives, all
to no avail. Hopefully someone who knows way more about postgresql than
me (not hard) can help me out here.

We are migrating our database from MS-SQL to Postgresql, and in the
process from a dual PIII-600 to a Pentium IV-2.4 based system, which to
my mind I'd have thought (bar the odd query needing a minor rewrite)
would lead to at the very least a slight performance improvement.

Alas what took MS-SQL 500ms to do is taking Postgresql about 140
seconds, obviously completely unacceptable. Even the simplest queries
seem to take far longer than I expected them to. :/

Yes I've got a load of indexes (which btw I have tried deleting and
recreating to no avail), in fact every field that uses an integer is
indexed in every table (I only do ORDER BYs and JOINs on integer
fields), run vacuum analyze countless times, etc, etc.

JOINs appear to be the worst offenders, with those on two tables or more
crippling the box for minutes at a time. CPU usage is always 92%+ when
trying to process these queries. Also when I run EXPLAIN on a query the
highest cost items in the execution plan are pretty much always Seq
Scans which don't use indexes.

The db has been transferred (complete with indexes and foreign keys)
using pgAdmin II and the pgMigration tools, both v1.3.82beta. Although
the migration tool claimed to encounter an error, and roll back the
whole migration process, from reading the logs (and viewing the
database) there was no actual error and everything appears to be there.

The database is not what I would consider large, only a few hundred MB.

I've changed the following (only) in the config file to give it more
memory which it seems to use, although none of this effects the speed:

sort_mem = 16384
shared_buffers = 16384

I'm running OpenBSD 3.1-stable, Postgresql 7.1.3 (from the ports
collection, although I've tried installing 7.2.3 and it has all the same
issues, so I went back to 7.1.3 just in case it fixed it), and this is a
completely clean brand new build with nothing else on it.

HELP!

Yours Unwhettedly,
Robert John Shepherd.

Editor
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