Richard Huxton <dev@archonet.com> writes:
> In the first, we match outer.clientnum to inner.clientnum, in the second
> it's "?column10?" - are you sure the query was identical in each case.
> I'm guessing the unidentified column in query 2 is the reason for the
> sort a couple of lines below it, which seems to take up a large chunk of
> time.
The "?column10?" is because EXPLAIN isn't excessively bright about
reporting references to outputs of lower plan nodes. (Gotta fix that
sometime.) The real point here is that the planner thought that a scan
plus sort would be faster than scanning an index that exactly matched
the sort order the Merge Join needed ... and it was wrong :-(
So this is just the usual sort of question of "are your stats up to
date, maybe you need to increase stats targets, or else play with
random_page_cost, etc" ...
regards, tom lane