On Sat, Apr 20, 2019 at 04:46:03PM -0400, Tom Lane wrote:
> Tomas Vondra <tomas.vondra@2ndquadrant.com> writes:
> > I think it's really a matter of underestimate, which convinces the planner
> > to hash the larger table. In this case, the table is 42GB, so it's
> > possible it actually works as expected. With work_mem = 4MB I've seen 32k
> > batches, and that's not that far off, I'd day. Maybe there are more common
> > values, but it does not seem like a very contrived data set.
>
> Maybe we just need to account for the per-batch buffers while estimating
> the amount of memory used during planning. That would force this case
> into a mergejoin instead, given that work_mem is set so small.
Do you mean by adding disable_cost if work_mem is so small that it's estimated
to be exceeded ?
Justin