Re: cost_hashjoin

On Mon, 2010-08-30 at 13:34 +0100, Greg Stark wrote:
> On Mon, Aug 30, 2010 at 10:18 AM, Simon Riggs <simon@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.comPostgreSQL Development, 24x7 Support, Training and Services