Include SubPlans when checking that a query is still distributed.

[colm-mchugh]

Include SubPlans when checking that a query is still distributed.

DESCRIPTION: tighten distributed plan check to cover distributed subplans.

The distributed_planner() hook needs to check, after calling standard_planner(), if the query still requires distributed planning - this was necessitated by issue #7782, #7783 where the citus tables in a query are optimized away by standard_planner(). However, issue #8313 exposed a case where we incorrectly flag a query as no long needing distributed planning; the query has
the following format:

SELECT t0.a1 as c1, 
       (SELECT xx FROM t1 LIMIT 1) as c2
FROM t0
WHERE true::bool

where the only citus table is t1. The scalar subquery is reduced to a read_intermediate_result() call after recursive planning of
CTEs and subqueries:

SELECT t0.a1 as c1, 
       (SELECT .. FROM read_intermediate_result() ... ) as c2
FROM t0

with the consequence that function CheckPostPlanDistribution() does not see any citus tables in the query plan and marks the query as not distributed. The fix enhances CheckPostPlanDistribution() to additionally look at the plan's subplan list for a read intermediate result call, and mark the query as needing distributed planning if one is found. The new check uses existing function IsReadIntermediateResultFunction() to detect the presence of a distributed subplan. This function is already used by intermediate_result_pruning.c to build a list of distributed subplan structs, so can be reused here to determine if
a query still needs distributed planning.

Fixes issue: #8313

[codecov]

Codecov Report

❌ Patch coverage is 61.90476% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.78%. Comparing base (87cd100) to head (e368b8c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8388      +/-   ##
==========================================
- Coverage   88.80%   88.78%   -0.03%     
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  Files         287      287              
  Lines       63239    63258      +19     
  Branches     7928     7931       +3     
==========================================
+ Hits        56158    56161       +3     
- Misses       4750     4765      +15     
- Partials     2331     2332       +1     

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[Copilot]

Pull request overview

This PR fixes issue #8313 where queries with Citus tables that were reduced to read_intermediate_result() calls after recursive planning of CTEs and subqueries were incorrectly flagged as not requiring distributed planning. The fix enhances the distributed planning check in distributed_planner() hook to inspect both the range table and the plan's subplan list for distributed relations.

• Enhanced distributed planning check to include subplan inspection for read_intermediate_result() function calls
• Made IsReadIntermediateResultFunction() publicly accessible for reuse
• Added comprehensive regression tests covering various scenarios with distributed subplans

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/backend/distributed/planner/distributed_planner.c	Added PlanContainsDistributedSubPlanRTE() function to check for distributed subplans and updated CheckPostPlanDistribution() to use it
src/backend/distributed/planner/multi_logical_planner.c	Changed IsReadIntermediateResultFunction() from static to public visibility
src/include/distributed/multi_logical_planner.h	Added declaration for the newly exported IsReadIntermediateResultFunction()
src/test/regress/sql/subquery_in_where.sql	Added test cases for queries with distributed subplans including minimal repro and variants with redundant WHERE clauses
src/test/regress/expected/subquery_in_where.out	Added expected output for the new test cases verifying correct results

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[colm-mchugh]

Note to reviewer: this change is not relevant to the fix, but this regress failed flakyness tests because of differences in the order of the results of this query. The ORDER BY stabilizes that.

[imranzaheer612]

@colm-mchugh looks like this is also an related issue: #8312

[colm-mchugh]

@colm-mchugh looks like this is also an related issue: #8312

Yes it does, and the PR also fixes #8312, from testing locally. Thanks for pointing out!