Re: Questions on query planner, join types, and work_mem

Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> Excerpts from Tom Lane's message of mar jul 27 20:05:02 -0400 2010:
>> Well, the issue you're hitting is that the executor is dividing the
>> query into batches to keep the size of the in-memory hash table below
>> work_mem. The planner should expect that and estimate the cost of
>> the hash technique appropriately, but seemingly it's failing to do so.

> Hmm, I wasn't aware that hash joins worked this way wrt work_mem. Is
> this visible in the explain output?

As of 9.0, any significant difference between "Hash Batches" and
"Original Hash Batches" would be a cue that the planner blew the
estimate. For Peter's problem, we're just going to have to look
to see if the estimated cost changes in a sane way between the
small-work_mem and large-work_mem cases.

regards, tom lane