On Wed, 2008-05-28 at 06:08 -0400, Gregory Stark wrote: > "Tom Lane" <[EMAIL PROTECTED]> writes: > > > (Although that argument might not hold water for a bulk seqscan: you'll > > have hinted all the tuples and then very possibly throw the page away > > immediately. > > That seems like precisely the case where we don't want to dirty the buffer.
(1) > > So counting the hints and eventually deciding we did > > enough to justify dirtying the page might be worth doing.) > > What if we counted how many hint bits were *not* set? I feel like the goal > should be to dirty the buffer precisely once when all the bits can be set. (2) Agreed. I think the difficulty is that (1) and (2) are contradictory goals, and since those conditions frequently occur together, cause conflict. When we fully scan a buffer this will result in 1 or more actual clog lookups, L. L is often less than the number of tuples on the page because of the single-item xid cache. If L = 1 then there is a high probability that when we do a seq scan the clog blocks will be cached also, so although we do a 1 clog lookup per table block we would seldom do clog I/O during a SeqScan. So what I tried to say in a previous post was that if L > 1 then we should dirty the buffer because the single-item cache becomes less-effective and we may need to access other clog blocks, that may result in clog I/O. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
