On Tuesday 04 September 2007 11:27:07 rafael@akyasociados.com.ar wrote:
> Hello everyone:
>
> I wanted to ask you about how the VACUUM ANALYZE works. is it possible
> that something can happen in order to reset its effects forcing to execute
> the VACUUM ANALYZE comand again? i am asking this because i am struggling
> with a query which works ok after i run a VACUUM ANALYZE, however,
> sudennly, it starts to take forever (the execution of the query) until i
> make another VACUUM ANALYZE, and so on ...
> I'd like to point that i am a novice when it comes to non basic
> postgresql performance related stuff.
>
> Thank you all in advance
>
> Rafael
>
>
>
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Rafael;
Vacuum Analyze performs 2 tasks at once.
1) Vacuum - this analyzes the table pages and sets appropriate dead row space
(those from old updates or deletes that are not possibly needed by any
existing transactions) as such that the db can re-use (over-write) that
space.
2) Analyze - Like an Oracle compute stats, updates the system catalogs with
current table stat data.
The Vacuum will improve queries since the dead space can be re-used and any
dead space if the table you are having issues with is a high volume table
then the solution is generally to run vacuum more often - I've seen tables
that needed a vacuum every 5 minutes due to significant sustained churn.
The Analyze of course is key for the planner, if the table is growing rapidly
then running analyze more often will help, if however there is lots of churn
but little change in the data (i.e. lots of inserts followed by delete's of
the same rows) then a straight vacuum is probably what you need. If the data
is changing rapidly then bumping up the default_statistics_target value may
help - you can bump the default_statistics_target for a single table in the
pg_autovacuum system catalog table.
Hope this helps...
/Kevin