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Re: Slow INSERT

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Michal Taborsky <michal(at)taborsky(dot)cz>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Slow INSERT
Date: 2004-06-29 15:22:48
Message-ID: 29835.1088522568@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-performance
Michal Taborsky <michal(at)taborsky(dot)cz> writes:
> I ran some tests to support this hypothesis. Every 500th insert is a tad 
> slower, but it is insignificant (normally the INSERT lasts 1.5ms, every 
> 500th is 9ms). During my tests (10 runs of 1000 INSERTS) I had 
> experienced only one "slow" insert (2000ms). It is clearly caused by 
> other processes running on this server, but such degradation of 
> performance is highly suspicious, because the server very rarely goes 
> over load 1.0.

Actually, the simpler theory is that the slowdown is caused by
background checkpoint operations.  Now a checkpoint would slow
*everything* down not only this one insert, so maybe that's not
the right answer either, but it's my next idea.  You could check
this to some extent by manually issuing a CHECKPOINT command and
seeing if you get an insert hiccup.  Note though that closely
spaced checkpoints will have less effect, because less I/O will
be triggered when not much has changed since the last one.  So
you'd want to wait a bit between experiments.

			regards, tom lane

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