>> For the first 1-2 days of running perfomance is excellent. But
>> after that, speed began to decrease. And after a week of
>> operation, perfomance falls 8-10 times, than at first day of
>> using.
>>
>> I'm doing vacuum periodically (once a hour), but perfomance
>> still falls down.
It sounds to me like you may be running into index growth problems.
VACUUM is presently not good about shrinking indexes. If you drop
and recreate the indexes used by your most important queries, does
the performance go back to where it was?
>> Why this situation occures? May be I must use "VACUUM ANALYSE"
>> instead of VACUUM?
Occasional VACUUM ANALYZEs are a good idea.
regards, tom lane
