Paul Tillotson <spam1011@adelphia.net> writes:
> For the following query, postgres is running the IN subquery over and
> over again (once for each row scanned in the parent table.)
> I would have expected it to run the whole query once and create a hash
> which would then be probed once for every row scanned in the parent
> table. I assumed that it was not doing so because it thought that the
> resulting hash table would exceed sort_mem,
Hardly likely, considering it's estimating only 296 rows in the subquery
output. My bet is that you've chosen a datatype whose comparisons are
not hashable (like char(n)). What is the datatype of parentid in these
tables, anyway?
regards, tom lane