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Tom Lane 4be058fe9e In the planner, replace an empty FROM clause with a dummy RTE. ... The fact that "SELECT expression" has no base relations has long been a thorn in the side of the planner. It makes it hard to flatten a sub-query that looks like that, or is a trivial VALUES() item, because the planner generally uses relid sets to identify sub-relations, and such a sub-query would have an empty relid set if we flattened it. prepjointree.c contains some baroque logic that works around this in certain special cases --- but there is a much better answer. We can replace an empty FROM clause with a dummy RTE that acts like a table of one row and no columns, and then there are no such corner cases to worry about. Instead we need some logic to get rid of useless dummy RTEs, but that's simpler and covers more cases than what was there before. For really trivial cases, where the query is just "SELECT expression" and nothing else, there's a hazard that adding the extra RTE makes for a noticeable slowdown; even though it's not much processing, there's not that much for the planner to do overall. However testing says that the penalty is very small, close to the noise level. In more complex queries, this is able to find optimizations that we could not find before. The new RTE type is called RTE_RESULT, since the "scan" plan type it gives rise to is a Result node (the same plan we produced for a "SELECT expression" query before). To avoid confusion, rename the old ResultPath path type to GroupResultPath, reflecting that it's only used in degenerate grouping cases where we know the query produces just one grouped row. (It wouldn't work to unify the two cases, because there are different rules about where the associated quals live during query_planner.) Note: although this touches readfuncs.c, I don't think a catversion bump is required, because the added case can't occur in stored rules, only plans. Patch by me, reviewed by David Rowley and Mark Dilger Discussion: https://postgr.es/m/15944.1521127664@sss.pgh.pa.us		7 years ago
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backend	In the planner, replace an empty FROM clause with a dummy RTE.	7 years ago
bin	psql: Remove unused tab completion query	7 years ago
common	Use perfect hashing, instead of binary search, for keyword lookup.	7 years ago
fe_utils	pgbench: add \cset and \gset commands	7 years ago
include	In the planner, replace an empty FROM clause with a dummy RTE.	7 years ago
interfaces	Remove _configthreadlocale() calls in ecpg test suite.	7 years ago
makefiles	Ensure link commands list *.o files before LDFLAGS.	7 years ago
pl	Change function call information to be variable length.	7 years ago
port	Use our own getopt() on OpenBSD.	7 years ago
template	Yet further rethinking of build changes for macOS Mojave.	7 years ago
test	In the planner, replace an empty FROM clause with a dummy RTE.	7 years ago
timezone	Replace @postgresql.org with @lists.postgresql.org for mailinglists	7 years ago
tools	Change function call information to be variable length.	7 years ago
tutorial	SQL comment: remove extra word in heading comment	7 years ago
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Makefile	Fix partial-build problems introduced by having more generated headers.	8 years ago
Makefile.global.in	fix typo	7 years ago
Makefile.shlib	Ensure static libraries have correct mod time even if ranlib messes it up.	7 years ago
nls-global.mk	Replace @postgresql.org with @lists.postgresql.org for mailinglists	7 years ago