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Tom Lane 41efb83408 Move resolution of AlternativeSubPlan choices to the planner. 
When commit bd3daddaf introduced AlternativeSubPlans, I had some
ambitions towards allowing the choice of subplan to change during
execution.  That has not happened, or even been thought about, in the
ensuing twelve years; so it seems like a failed experiment.  So let's
rip that out and resolve the choice of subplan at the end of planning
(in setrefs.c) rather than during executor startup.  This has a number
of positive benefits:

* Removal of a few hundred lines of executor code, since
AlternativeSubPlans need no longer be supported there.

* Removal of executor-startup overhead (particularly, initialization
of subplans that won't be used).

* Removal of incidental costs of having a larger plan tree, such as
tree-scanning and copying costs in the plancache; not to mention
setrefs.c's own costs of processing the discarded subplans.

* EXPLAIN no longer has to print a weird (and undocumented)
representation of an AlternativeSubPlan choice; it sees only the
subplan actually used.  This should mean less confusion for users.

* Since setrefs.c knows which subexpression of a plan node it's
working on at any instant, it's possible to adjust the estimated
number of executions of the subplan based on that.  For example,
we should usually estimate more executions of a qual expression
than a targetlist expression.  The implementation used here is
pretty simplistic, because we don't want to expend a lot of cycles
on the issue; but it's better than ignoring the point entirely,
as the executor had to.

That last point might possibly result in shifting the choice
between hashed and non-hashed EXISTS subplans in a few cases,
but in general this patch isn't meant to change planner choices.
Since we're doing the resolution so late, it's really impossible
to change any plan choices outside the AlternativeSubPlan itself.

Patch by me; thanks to David Rowley for review.

Discussion: https://postgr.es/m/1992952.1592785225@sss.pgh.pa.us
 		2020-09-27 12:51:28 -04:00
config Rename configure.in to configure.ac 2020-07-24 10:42:08 +02:00
contrib Remove custom memory allocation layer in pgcrypto 2020-09-25 10:25:55 +09:00
doc Add new 'old_snapshot' contrib module. 2020-09-24 13:55:47 -04:00
src Move resolution of AlternativeSubPlan choices to the planner. 2020-09-27 12:51:28 -04:00
.dir-locals.el Make Emacs perl-mode indent more like perltidy. 2019-01-13 11:32:31 -08:00
.editorconfig Add .editorconfig 2019-12-18 09:13:13 +01:00
.gitattributes gitattributes: Add new file 2019-11-12 08:13:55 +01:00
.gitignore Support for optimizing and emitting code in LLVM JIT provider. 2018-03-22 11:05:22 -07:00
COPYRIGHT Update copyrights for 2020 2020-01-01 12:21:45 -05:00
GNUmakefile.in Make install-tests target work with vpath builds 2020-05-31 18:33:00 -04:00
HISTORY Canonicalize some URLs 2020-02-10 20:47:50 +01:00
Makefile Don't unset MAKEFLAGS in non-GNU Makefile. 2019-06-25 09:36:21 +12:00
README Canonicalize some URLs 2020-02-10 20:47:50 +01:00
README.git Canonicalize some URLs 2020-02-10 20:47:50 +01:00
aclocal.m4 Fix configure's AC_CHECK_DECLS tests to work correctly with clang. 2018-11-19 12:01:47 -05:00
configure Split Makefile symbol CFLAGS_VECTOR into two symbols. 2020-09-06 21:28:16 -04:00
configure.ac Split Makefile symbol CFLAGS_VECTOR into two symbols. 2020-09-06 21:28:16 -04:00

README 

PostgreSQL Database Management System
=====================================

This directory contains the source code distribution of the PostgreSQL
database management system.

PostgreSQL is an advanced object-relational database management system
that supports an extended subset of the SQL standard, including
transactions, foreign keys, subqueries, triggers, user-defined types
and functions.  This distribution also contains C language bindings.

PostgreSQL has many language interfaces, many of which are listed here:

	https://www.postgresql.org/download/

See the file INSTALL for instructions on how to build and install
PostgreSQL.  That file also lists supported operating systems and
hardware platforms and contains information regarding any other
software packages that are required to build or run the PostgreSQL
system.  Copyright and license information can be found in the
file COPYRIGHT.  A comprehensive documentation set is included in this
distribution; it can be read as described in the installation
instructions.

The latest version of this software may be obtained at
https://www.postgresql.org/download/.  For more information look at our
web site located at https://www.postgresql.org/.