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David Rowley a14a583292 Add additional regression tests for select_active_windows
During the development of 728202b63, which was aimed at reducing the
number of sorts required to evaluate multiple window functions with
different WindowClause definitions, the code written sorted the
WindowClauses in reverse tleSortGroupRef order.  There appears to be no
discussion in the thread which was opened to discuss the development of
this patch and no comments mentioning the fact that having the
WindowClauses in reverse tleSortGroupRef order makes it more likely that
the final WindowClause to be evaluated will provide presorted input to
the query's DISTINCT or ORDER BY clause.  The reason for this is that the
tleSortGroupRef indexes are assigned for the DISTINCT and ORDER BY clauses
before they are for the WindowClauses PARTITION BY and ORDER BY clauses.
Putting the WindowClause with the lowest tleSortGroupRef last means that
it's more likely that no additional sorting is required for the query's
DISTINCT or ORDER BY clause.

All we're doing here is adding some tests and a comment to help ensure
that remains true and that we don't accidentally forget to consider this
again should we ever rewrite that code.

Author: Ankit Kumar Pandey, David Rowley
Discussion: https://postgr.es/m/CAApHDvq=g2=ny59f1bvwRVvupsgPHK-KjLPBsSL25fVuGZ4idQ@mail.gmail.com
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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/.