Stabilize multi_insert_select expected: accept unqualified columns in WHERE … IS NOT NULL (PG15–PG18)

[m3hm3t]

DESCRIPTION: Stabilize multi_insert_select expected: accept unqualified columns in WHERE … IS NOT NULL

fixes #8133

Context

• With PG18, ruleutils adds a GROUP RTE and improves column-name dedup. As a side-effect, Vars that point at the GROUP RTE print as unqualified column names even when varprefix is true.
• In Citus’ vendored ruleutils_18.c we already flattened GROUP Vars in targetList and havingQual, but not in jointree->quals.
• For queries like INSERT … SELECT … GROUP BY …, Citus injects an implicit null-guard on the group key in the WHERE clause. Because that Var was still referencing the GROUP RTE, the deparser emitted WHERE (user_id IS NOT NULL) instead of WHERE (raw_events_first.user_id IS NOT NULL), causing regress diffs only in grouped SELECTs.
• Related upstream change: PostgreSQL commit 52c707483ce4d0161127e4958d981d1b5655865e (ruleutils column-name de-dup / GROUP RTE exposure).

What changed

• Added an alternative expected file src/test/regress/expected/multi_insert_select_0.out to keep CI green across mixed environments where the qualified form may still be produced.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.89%. Comparing base (91dfbd9) to head (2cbed2d).
⚠️ Report is 1 commits behind head on m3hm3t/pg18_beta_confs.

Additional details and impacted files

@@                   Coverage Diff                   @@
##           m3hm3t/pg18_beta_confs    #8139   +/-   ##
=======================================================
  Coverage                   88.88%   88.89%           
=======================================================
  Files                         287      287           
  Lines                       63137    63137           
  Branches                     7938     7938           
=======================================================
+ Hits                        56119    56124    +5     
+ Misses                       4681     4679    -2     
+ Partials                     2337     2334    -3     

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[Copilot]

Pull Request Overview

This PR adds PostgreSQL 18 support to Citus while stabilizing the multi_insert_select test to handle differences in column qualification behavior between PostgreSQL versions. The changes prepare Citus for compatibility with PostgreSQL 18 beta and address test regression issues caused by upstream changes in PostgreSQL's ruleutils column naming.

Key changes:

• Add PostgreSQL 18 support across build configurations and CI pipelines
• Update version compatibility checks to include PostgreSQL 18
• Switch default PostgreSQL version to 18beta3 in development environment

Reviewed Changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
configure.ac	Extends PostgreSQL version compatibility check to include version 18
configure	Generated configure script with updated version compatibility logic
.github/workflows/packaging-test-pipelines.yml	Improves regex pattern for extracting PostgreSQL versions from workflow files
.github/workflows/build_and_test.yml	Adds PostgreSQL 18 test configurations and updates version parameters
.devcontainer/Dockerfile	Adds PostgreSQL 18beta3 build stage and sets it as default version

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