David Rowley f31aad9b07 Fix query jumbling to account for NULL nodes
Previously NULL nodes were ignored.  This could cause issues where the
computed query ID could match for queries where fields that are next to
each other in their Node struct where one field was NULL and the other
non-NULL.  For example, the Query struct had distinctClause and sortClause
next to each other.  If someone wrote;

SELECT DISTINCT c1 FROM t;

and then;

SELECT c1 FROM t ORDER BY c1;

these would produce the same query ID since, in the first query, we
ignored the NULL sortClause and appended the jumble bytes for the
distictClause.  In the latter query, since we did nothing for the NULL
distinctClause then jumble the non-NULL sortClause, and since the node
representation stored is the same in both cases, the query IDs were
identical.

Here we fix this by always accounting for NULL nodes by recording that
we saw a NULL in the jumble buffer.  This fixes the issue as the order that
the NULL is recorded isn't the same in the above two queries.

Author: Bykov Ivan <i.bykov@modernsys.ru>
Author: Michael Paquier <michael@paquier.xyz>
Author: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/aafce7966e234372b2ba876c0193f1e9%40localhost.localdomain
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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.

Copyright and license information can be found in the file COPYRIGHT.

General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.

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

Description
Mirror of the official PostgreSQL GIT repository. Note that this is just a *mirror* - we don't work with pull requests on github. To contribute, please see https://wiki.postgresql.org/wiki/Submitting_a_Patch
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