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.
Aha! This is really the case. I've let the test run and issued manual CHECKPOINT command. The command itself took about 3 secs and during that time I had some slow INSERTS. So we know the reason.
I've read the discussion in "Trying to minimize the impact of checkpoints" thread and I get it, that there is nothing I can do about it. Well, we'll have to live with that, at least until 7.5.
Thanks of the help all the same.
-- Michal Taborsky http://www.taborsky.cz
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