Matthew <matthew@flymine.org> writes:
> ... For this query, Postgres would perform a nested loop,
> iterating over all rows in the small table, and doing a hundred index
> lookups in the big table. This completed very quickly. However, adding the
> LIMIT meant that suddenly a merge join was very attractive to the planner,
> as it estimated the first row to be returned within milliseconds, without
> needing to sort either table.
> The problem is that Postgres didn't know that the first hit in the big
> table would be about half-way through, after doing a index sequential scan
> for half a bazillion rows.
Hmm. IIRC, there are smarts in there about whether a mergejoin can
terminate early because of disparate ranges of the two join variables.
Seems like it should be straightforward to fix it to also consider
whether the time-to-return-first-row will be bloated because of
disparate ranges. I'll take a look --- but it's probably too late
to consider this for 8.3.
regards, tom lane
