On Mon, 2010-08-30 at 13:34 +0100, Greg Stark wrote: > On Mon, Aug 30, 2010 at 10:18 AM, Simon Riggs <si...@2ndquadrant.com> wrote: > > cost_hashjoin() has some treatment of what occurs when numbatches > 1 > > but that additional cost is not proportional to numbatches. > > Because that's not how our hash batching works. We generate two temp > files for each batch, one for the outer and one for the inner. So if > we're batching then every tuple of both the inner and outer tables > (except for ones in the first batch) need to be written once and read > once regardless of the number of batches.
Thanks for explaining. For some reason I thought we rewound the outer at the start of each batch, which is better for avoiding cache spoiling. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Training and Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
