Robert Haas <robertmhaas@gmail.com> writes:
> Of course, if we can postpone sizing the hash table until after the
> input size is known, as you suggest, then that would be better still
> (but not back-patch material).
AFAICS, it works that way today as long as the hash fits in memory
(ie, single-batch). We load into a possibly seriously undersized hash
table, but that won't matter for performance until we start probing it.
At the conclusion of loading, MultiExecHash will call
ExecHashIncreaseNumBuckets which will re-hash into a better-sized hash
table. I doubt this can be improved on much.
It would be good if we could adjust the numbuckets choice at the
conclusion of the input phase for the multi-batch case as well.
The code appears to believe that wouldn't work, but I'm not sure if
it's right about that, or how hard it'd be to fix if so.
regards, tom lane