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1349d279added query planner support to allow more efficient execution of aggregate functions which have an ORDER BY or a DISTINCT clause. Prior to that commit, the planner would only request that the lower planner produce a plan with the order required for the GROUP BY clause and it would be left up to nodeAgg.c to perform the final sort of records within each group so that the aggregate transition functions were called in the correct order. Now that the planner requests the lower planner produce a plan with the GROUP BY and the ORDER BY / DISTINCT aggregates in mind, there is the possibility that the planner chooses a plan which could be less efficient than what would have been produced before1349d279. While developing1349d279, I had in mind that Incremental Sort would help us in cases where an index exists only on the GROUP BY column(s). Incremental Sort would just replace the implicit tuplesorts which are being performed in nodeAgg.c. However, because the planner has the flexibility to instead choose a plan which just performs a full sort on both the GROUP BY and ORDER BY / DISTINCT aggregate columns, there is potential for the planner to make a bad choice. The costing for Incremental Sort is not perfect as it assumes an even distribution of rows to sort within each sort group. Here we add an escape hatch in the form of the enable_presorted_aggregate GUC. This will allow users to get the pre-PG16 behavior in cases where they have no other means to convince the query planner to produce a plan which only sorts on the GROUP BY column(s). Discussion: https://postgr.es/m/CAApHDvr1Sm+g9hbv4REOVuvQKeDWXcKUAhmbK5K+dfun0s9CvA@mail.gmail.com
# Generating dummy probes If Postgres isn't configured with dtrace enabled, we need to generate dummy probes for the entries in probes.d, that do nothing. This is accomplished in Unix via the sed script `Gen_dummy_probes.sed`. We used to use this in MSVC builds using the perl utility `psed`, which mimicked sed. However, that utility disappeared from Windows perl distributions and so we converted the sed script to a perl script to be used in MSVC builds. We still keep the sed script as the authoritative source for generating these dummy probes because except on Windows perl is not a hard requirement when building from a tarball. So, if you need to change the way dummy probes are generated, first change the sed script, and when it's working generate the perl script. This can be accomplished by using the perl utility s2p. s2p is no longer part of the perl core, so it might not be on your system, but it is available on CPAN and also in many package systems. e.g. on Fedora it can be installed using `cpan App::s2p` or `dnf install perl-App-s2p`. The Makefile contains a recipe for regenerating Gen_dummy_probes.pl, so all you need to do is once you have s2p installed is `make Gen_dummy_probes.pl` Note that in a VPATH build this will generate the file in the vpath tree, not the source tree.