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Tom Lane f4c7c410ee Revert "Optimize order of GROUP BY keys". ... This reverts commit db0d67db24 and several follow-on fixes. The idea of making a cost-based choice of the order of the sorting columns is not fundamentally unsound, but it requires cost information and data statistics that we don't really have. For example, relying on procost to distinguish the relative costs of different sort comparators is pretty pointless so long as most such comparator functions are labeled with cost 1.0. Moreover, estimating the number of comparisons done by Quicksort requires more than just an estimate of the number of distinct values in the input: you also need some idea of the sizes of the larger groups, if you want an estimate that's good to better than a factor of three or so. That's data that's often unknown or not very reliable. Worse, to arrive at estimates of the number of calls made to the lower-order-column comparison functions, the code needs to make estimates of the numbers of distinct values of multiple columns, which are necessarily even less trustworthy than per-column stats. Even if all the inputs are perfectly reliable, the cost algorithm as-implemented cannot offer useful information about how to order sorting columns beyond the point at which the average group size is estimated to drop to 1. Close inspection of the code added by db0d67db2 shows that there are also multiple small bugs. These could have been fixed, but there's not much point if we don't trust the estimates to be accurate in-principle. Finally, the changes in cost_sort's behavior made for very large changes (often a factor of 2 or so) in the cost estimates for all sorting operations, not only those for multi-column GROUP BY. That naturally changes plan choices in many situations, and there's precious little evidence to show that the changes are for the better. Given the above doubts about whether the new estimates are really trustworthy, it's hard to summon much confidence that these changes are better on the average. Since we're hard up against the release deadline for v15, let's revert these changes for now. We can always try again later. Note: in v15, I left T_PathKeyInfo in place in nodes.h even though it's unreferenced. Removing it would be an ABI break, and it seems a bit late in the release cycle for that. Discussion: https://postgr.es/m/TYAPR01MB586665EB5FB2C3807E893941F5579@TYAPR01MB5866.jpnprd01.prod.outlook.com		4 years ago
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backend	Revert "Optimize order of GROUP BY keys".	4 years ago
bin	Fix tab-completion after commit 790bf615dd	4 years ago
common	Change some errdetail() to errdetail_internal()	4 years ago
fe_utils	Revert 56-bit relfilenode change and follow-up commits.	4 years ago
include	Revert "Optimize order of GROUP BY keys".	4 years ago
interfaces	Adjust PQsslAttributeNames() to match PQsslAttribute().	4 years ago
makefiles	windows: remove date from version number in win32ver.rc	4 years ago
pl	Introduce GUC_NO_RESET flag.	4 years ago
port	windows: Set UMDF_USING_NTSTATUS globally, include ntstatus.h	4 years ago
template	Move darwin sysroot determination into separate file	4 years ago
test	Revert "Optimize order of GROUP BY keys".	4 years ago
timezone	meson: Add initial version of meson based build system	4 years ago
tools	ci: Add hint about downloadable logs to README	4 years ago
tutorial	Update copyright for 2022	4 years ago
.gitignore	…
DEVELOPERS	…
Makefile	…
Makefile.global.in	Split TESTDIR into TESTLOGDIR and TESTDATADIR	4 years ago
Makefile.shlib	aix: No need to use mkldexport when we want to export all symbols	4 years ago
meson.build	meson: Add initial version of meson based build system	4 years ago
nls-global.mk	Fix for make unportability	4 years ago