"Markus Wollny" <Markus.Wollny@computec.de> writes:
>> ... What I find interesting though is
>> that the plain index scan in 8.0 is so enormously cheaper
>> than it's estimated to be. Perhaps the answer table in your
>> 8.0 installation is almost perfectly ordered by session_id?
> Not quite - there may be several concurrent sessions at any one time, but ordinarily the answers for one session-id
wouldbe quite close together, in a lot of cases even in perfect sequence, so "almost perfectly" might be a fair
description,depending on the exact definition of "almost" :)
Could we see the pg_stats row for answer.session_id in both 8.0 and 8.1?
> I had set random_page_cost to 1.4 already, so I doubt that it would do much good to further reduce the value -
readingthe docs and the suggestions for tuning I would have thought that I should actually consider increasing this
valuea bit, as not all of my data will fit in memory any more. Do you nevertheless want me to try what happens if I
reducerandom_page_cost even further?
No, that's probably quite low enough already ...
regards, tom lane