Fix 0-Task Plans in Single-Shard Router When Updating a Local Table with Reference Table in Subquery

[m3hm3t]

This PR fixes an issue #7891 in the Citus planner where an UPDATE on a local table with a subquery referencing a reference table could produce a 0-task plan. Historically, the planner sometimes failed to detect that both the target and referenced tables were effectively “local,” assigning INVALID_SHARD_ID and yielding a no-op plan.

Root Cause

• In the Citus router logic (PlanRouterQuery), we relied on shardId to determine whether a query should be routed to a single shard.
• If shardId == INVALID_SHARD_ID, but we also had not marked the query as a “local table modification,” the code path would produce zero tasks.
• Local + reference tables do not require multi-shard routing. Failing to detect this “purely local” scenario caused Citus to incorrectly route to zero tasks.

Changes

Enhanced Local Table Detection

• Updated IsLocalTableModification and related checks to consider both local and reference tables as “local” for planning, preventing the 0-task scenario.
• Expanded ContainsOnlyLocalOrReferenceTables to return true if there are no fully distributed tables in the query.

Added Regress Test

• Introduced a new regress test (issue_7891.sql) which reproduces the scenario.
• Verifies we get a valid single- or local-task plan rather than a 0-task plan.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.46%. Comparing base (c55bc8c) to head (6fd8a85).
Report is 22 commits behind head on release-13.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-13.0    #7897      +/-   ##
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- Coverage         89.48%   89.46%   -0.02%     
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  Files               276      276              
  Lines             60063    60009      -54     
  Branches           7524     7516       -8     
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- Hits              53747    53687      -60     
- Misses             4166     4173       +7     
+ Partials           2150     2149       -1     

[colm-mchugh]

Lgtm, thanks for accommodating the test requests.