On Fri, Aug 12, 2011 at 8:28 AM, Simon Riggs <si...@2ndquadrant.com> wrote: > On Fri, Aug 12, 2011 at 1:14 PM, Robert Haas <robertmh...@gmail.com> wrote: >> On Fri, Aug 12, 2011 at 4:33 AM, Simon Riggs <si...@2ndquadrant.com> wrote: >>> You're missing an important point. The SeqScan is measurably faster >>> when using the ring buffer because of the effects of L2 cacheing on >>> the buffers. >> >> I hadn't thought of that, but I think that's only true if the relation >> won't fit in shared_buffers (or whatever portion of shared_buffers is >> reasonably available, given the other activity on the machine). In >> this particular case, it's almost 20% faster if the relation is all in >> shared_buffers; I tested it. I think what's going on here is that >> initscan() has a heuristic that tries to use a BufferAccessStrategy if >> the relation is larger than 1/4 of shared_buffers. That's probably a >> pretty good heuristic in general, but in this case I have a relation >> which just so happens to be 27.9% of shared_buffers but will still >> fit. As you say below, that may not be typical in real life, although >> there are probably data warehousing systems where it's normal to have >> only one big query running at a time. > > I think there are reasonable arguments to make > > * prefer_cache = off (default) | on a table level storage parameter, > =on will disable the use of BufferAccessStrategy > > * make cache_spoil_threshold a parameter, with default 0.25
Yeah, something like that might make sense. Of course, a completely self-tuning system would be better, but I'm not sure we're going to find one of those. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
