Bruce Momjian wrote:
>>Oracle partially mitigates the second effect by refusing to trash its
>>entire buffer cache on any given sequential scan. Or so I've been told
>>by people who know Oracle well. A repeat of the sequential scan will
>>still have to reread the entire table but that's true anyway if the
>>table's at least one block longer than available cache.
>>
>
> That is on our TODO list, at least.
I didn't realize this, it's good news. (I don't follow PG development
closely these days).
BTW overall I think the cost-estimating portion of the PG optimizer does
about as well as Oracle's. Oracle is a lot smarter about doing
transformations of certain types of queries (turning "scalar in (select
...)" into something akin to an "exists") but of course this has nothing
to do with estimating the cost of index vs. sequential scans.
--
Don Baccus
Portland, OR
http://donb.photo.net, http://birdnotes.net, http://openacs.org