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a80818605e
Quite a few matching operators such as JSONB's @> used "contsel" and "contjoinsel" as their selectivity estimators. That was a bad idea, because (a) contsel is only a stub, yielding a fixed default estimate, and (b) that default is 0.001, meaning we estimate these operators as five times more selective than equality, which is surely pretty silly. There's a good model for improving this in ltree's ltreeparentsel(): for any "var OP constant" query, we can try applying the operator to all of the column's MCV and histogram values, taking the latter as being a random sample of the non-MCV values. That code is actually 100% generic, except for the question of exactly what default selectivity ought to be plugged in when we don't have stats. Hence, migrate the guts of ltreeparentsel() into the core code, provide wrappers "matchingsel" and "matchingjoinsel" with a more-appropriate default estimate, and use those for the non-geometric operators that formerly used contsel (mostly JSONB containment operators and tsquery matching). Also apply this code to some match-like operators in hstore, ltree, and pg_trgm, including the former users of ltreeparentsel as well as ones that improperly used contsel. Since commit 911e70207 just created new versions of those extensions that we haven't released yet, we can sneak this change into those new versions instead of having to create an additional generation of update scripts. Patch by me, reviewed by Alexey Bashtanov Discussion: https://postgr.es/m/12237.1582833074@sss.pgh.pa.us
The PostgreSQL contrib tree
---------------------------
This subtree contains porting tools, analysis utilities, and plug-in
features that are not part of the core PostgreSQL system, mainly
because they address a limited audience or are too experimental to be
part of the main source tree. This does not preclude their
usefulness.
User documentation for each module appears in the main SGML
documentation.
When building from the source distribution, these modules are not
built automatically, unless you build the "world" target. You can
also build and install them all by running "make all" and "make
install" in this directory; or to build and install just one selected
module, do the same in that module's subdirectory.
Some directories supply new user-defined functions, operators, or
types. To make use of one of these modules, after you have installed
the code you need to register the new SQL objects in the database
system by executing a CREATE EXTENSION command. In a fresh database,
you can simply do
CREATE EXTENSION module_name;
See the PostgreSQL documentation for more information about this
procedure.