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Re: PG8.2.1 choosing slow seqscan over idx scan

From: Tomas Vondra <tv(at)fuzzy(dot)cz>
To:
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: PG8.2.1 choosing slow seqscan over idx scan
Date: 2007-01-17 19:32:37
Message-ID: 45AE79D5.9080000@fuzzy.cz (view raw or flat)
Thread:
Lists: pgsql-performance
> That's about 32% dead rows.  Might be worth scheduling a vacuum full,
> but it's not like I was afraid it might be.  It looks to me like you
> could probably use a faster I/O subsystem in that machine though.
> 
> If the random page cost being lower fixes your issues, then I'd just run
> with it lower for now.  note that while lowering it may fix one query,
> it may break another.  Tuning pgsql, like any database, is as much art
> as science...

A nice feature of postgresql is the ability to log the 'slow queries'
(exceeding some time limit) - you can use it to compare the performance
of various settings. We're using it to 'detect' stupid SQL etc.

Just set it reasonably (the value depends on you), for example we used
about 500ms originally and after about two months of improvements we
lowered it to about 100ms.

You can analyze the log by hand, but about a year ago I've written a
tool to parse it and build a set of HTML reports with an overview and
details about each query) along with graphs and examples of queries.

You can get it here: http://opensource.pearshealthcyber.cz/

Just beware, it's written in PHP and it definitely is not perfect:

   (1) memory requirements (about 4x the size of the log)
   (2) not to fast (about 20mins of P4(at)3GHz for a 200MB log)
   (3) it requires a certain log format (see the page)

I did some improvements to the script recently, but forgot to upload it.
I'll do that tomorrow.

Tomas

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Next:From: Jeremy HaileDate: 2007-01-17 19:38:52
Subject: Re: PG8.2.1 choosing slow seqscan over idx scan
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Subject: Re: PG8.2.1 choosing slow seqscan over idx scan

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