Re: HashJoin order, hash the large or small table? Postgres likes to hash the big one, why?

Scott Carey <scott@richrelevance.com> writes:
> I consistently see HashJoin plans that hash the large table, and scan
> the small table.

Could we see a self-contained test case?  And what cost parameters are
you using, especially work_mem?

> This is especially puzzling in some cases where I have 30M rows in the big table and ~ 100 in the small... shouldn't
ithash the small table and scan the big one? 

Well, size of the table isn't the only factor; in particular, a highly
nonuniform distribution of the key value will inflate the cost estimate
for using a table on the inner size of the hash.  But the example you
show here seems a bit extreme.

            regards, tom lane