Christian Storm wrote:
>>At the moment, my rule of thumb is to check out the ANALYZE VERBOSE
>>messages to see if all table pages are being scanned.
>>
>> INFO: "mytable": scanned xxx of yyy pages, containing ...
>>
>>If xxx = yyy, then I keep statistics at the current level.
>>When xxx is way less than yyy, I increase the numbers a bit
>>and retry.
>>
>>It's probably primitive, but it seems to work well.
>
> What heuristic do you use to up the statistics for such a table?
No heuristics, just try and see.
For tables of ~ 10k pages, I set statistics to 100/200.
For ~ 100k pages, I set them to 500 or more.
I don't know the exact relation.
> Once you've changed it, what metric do you use to
> see if it helps or was effective?
I rerun an analyze and see the results... :-)
If you mean checking the usefulness, I can see it only
under heavy load, if particular db queries run in the order
of a few milliseconds.
If I see normal queries that take longer and longer, or
they even appear in the server's log (> 500 ms), then
I know an analyze is needed, or statistics should be set higher.
--
Cosimo