Re: pgsql: Teach tuplesort.c about "top N" sorting, in which only the first

Magnus Hagander <magnus@hagander.net> writes:
> Is there some way to see in the generated query plan if this optimisation
> is used?

If there's a SORT just below a LIMIT (that has a limit, ie it's not just
an OFFSET), then it's potentially used.  Whether it's actually used
depends on actual row counts and widths at runtime --- you'd have to
turn on trace_sort and look at the log output to determine that.

Also, if you want to experiment, you can compile with -DDEBUG_BOUNDED_SORT
to have a GUC variable optimize_bounded_sort that disables the new code
for comparison purposes.

            regards, tom lane