Iain wrote:
Yes, you are right but it wasn't the case this time, I have run the
explain plenty of
times with same results. I think that the reason was that I made a simple
VACUUM,
after a VACUUM FULL ANALYZE (1h!!) things are ok
It's reasonable to expect that a seq scan will perform faster after a full
vacuum, as the physical size and organization of the table has been changed.
I wouldn't expact a plain vacuum to have any tangible affect on performance,
for the better or for the worse..
A plain VACUUM (without ANALYZE) will change the layout of the data without refreshing the optimizer information, so that anything that DOES use the optimizer will often be negatively affected; VACUUM ANALYZE does a vacuum AND refreshes the optimizer information.
I usually run VACUUM ANALYZE on an hourly basis on our production system, and it's fast enough and unobtrusive enough that I can't tell that it's running (except by looking at the log, of course).
I'd like to know more about the possibility of plain vacuums harming
performance. This is the first I've heard of it. Vacuum full is not always
an option in a production environment.
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