RE: Query ID Calculation Fix for DISTINCT / ORDER BY and LIMIT / OFFSET

Hello!

Last time, I forgot to attach the patches.

The problem still persists in the 17.3 release.

Solution One
============

The simplest way to fix the problem is to place the scalar field used in the query ID calculation

between similar subnodes.
A patch for this solution is attached below (0001-Query-ID-Calculation-Fix-Variant-A.patch).

Solution Two
============

Alternatively, we can change the hash sum when we encounter an empty node.
This approach may impact performance but will protect us from such errors in the future.
A patch for this solution is attached below (0001-Query-ID-Calculation-Fix-Variant-B.patch).

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SELECT version();

                                             version                                             

-------------------------------------------------------------------------------------------------

PostgreSQL 17.3 on x86_64-pc-linux-gnu, compiled by gcc (Debian 12.2.0-14) 12.2.0, 64-bit

SET compute_query_id = on;

/* LIMIT / OFFSET */

EXPLAIN (VERBOSE) SELECT "oid" FROM pg_class LIMIT 1;

                                 QUERY PLAN                                

----------------------------------------------------------------------------

Limit  (cost=0.00..0.04 rows=1 width=4)

   Output: oid

   ->  Seq Scan on pg_catalog.pg_class  (cost=0.00..18.15 rows=415 width=4)

         Output: oid

Query Identifier: 5185884322440896420

EXPLAIN (VERBOSE) SELECT "oid" FROM pg_class OFFSET 1;

                                 QUERY PLAN                                

----------------------------------------------------------------------------

Limit  (cost=0.04..18.15 rows=414 width=4)

   Output: oid

   ->  Seq Scan on pg_catalog.pg_class  (cost=0.00..18.15 rows=415 width=4)

         Output: oid

Query Identifier: 5185884322440896420

/* DISTINCT / ORDER BY */

EXPLAIN (VERBOSE) SELECT DISTINCT "oid" FROM pg_class;

                                                 QUERY PLAN                                                

------------------------------------------------------------------------------------------------------------

Unique  (cost=0.27..23.54 rows=415 width=4)

   Output: oid

   ->  Index Only Scan using pg_class_oid_index on pg_catalog.pg_class  (cost=0.27..22.50 rows=415 width=4)

         Output: oid

Query Identifier: 751948508603549510

EXPLAIN (VERBOSE) SELECT "oid" FROM pg_class ORDER BY "oid";

                                              QUERY PLAN                                             

------------------------------------------------------------------------------------------------------

Index Only Scan using pg_class_oid_index on pg_catalog.pg_class  (cost=0.27..22.50 rows=415 width=4)

   Output: oid

Query Identifier: 751948508603549510