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Re: average query performance measuring

From: Tomas Vondra <tv(at)fuzzy(dot)cz>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: average query performance measuring
Date: 2012-08-21 21:08:09
Message-ID: 5033F8B9.4080806@fuzzy.cz (view raw or flat)
Thread:
Lists: pgsql-performance
On 21.8.2012 20:35, Rick Otten wrote:
> I have a PostgreSQL 9.1 cluster.  Each node is serving around 1,000
> queries per second when we are at a ‘steady state’.
> 
> What I’d like to know is the average query time.  I’d like to see if
> query performance is consistent, or if environmental changes, or code
> releases, are causing it to drift, spike, or change.   I’d also like to
> be able to compare the (real) query performance on the different nodes.
> 
> I know I can put some sort of query wrapper at the application layer to
> gather and store timing info.  (I’m not sure yet how the application
> would know which node the query just ran on since we are using pgpool
> between the app and the db.)   I’d much rather get something directly
> out of each database node if I can.
> 
> Turning on statement logging crushes the database performance, so I
> don’t want to do that either.  (Not to mention I’d still have to parse
> the logs to get the data.)
> 
> It seems like we almost have everything we need to track this in the
> stats tables, but not quite.  I was hoping the folks on this list would
> have some tips on how to get query performance trends over time out of
> each node in my cluster.

As others already mentioned, the improvements in pg_stat_statements by
Peter Geoghean in 9.2 is the first thing you should look into I guess.
Especially if you're looking for per-query stats.

If you're looking for "global stats," you might be interested in an
extension I wrote a few months ago and collects query histogram. It's
available on pgxn.org: http://pgxn.org/dist/query_histogram/

The question is whether tools like this can give you reliable answers to
your questions - that depends on your workload (how much it varies) etc.

Tomas


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