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We've long had enable_hashagg to discourage hashed aggregation, but there was no equivalent for grouping that works by reading presorted input. That's handy when investigating planner cost misestimates, and as an escape hatch when a sorted grouping plan is chosen over a much cheaper hashed one. enable_groupagg (on by default) covers the GroupAggregate and Group nodes, the sort-based Unique step used for DISTINCT and semijoin unique-ification, and the sorted mode of SetOp. It isn't a hard switch; it only bumps disabled_nodes, so plans with no other choice are still produced. Several regression and module tests had been setting enable_sort off, and one enable_indexscan off, only to force a hashed plan. Use enable_groupagg there instead, where that was the real intent. In union.sql this also lets us test the hashed UNION path, which we had no way to reach before. Author: Tatsuro Yamada <yamatattsu@gmail.com> Co-authored-by: Richard Guo <guofenglinux@gmail.com> Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com> Reviewed-by: David Rowley <dgrowleyml@gmail.com> Discussion: https://postgr.es/m/CAOKkKFvYHSEsFazkrf9bRH14p-H27XMaqbZfRYjS6EHBruvZMQ@mail.gmail.com
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.