fix: handle nested arrays in UNNEST by checking for grouping in left query (#17949)

[zhang-arvin]

Description

Fixes #17949: UNNEST with nested arrays fails when the left side of the correlate contains GROUP BY aggregation.

Root Cause

DruidCorrelateUnnestRel.toDruidQuery() uses computeLeftRequiresSubquery() to determine whether to wrap the left query in a QueryDataSource. However, computeLeftRequiresSubquery() only checks the PartialDruidQuery stage (SCAN), but when UNNEST is pulled up above a GROUP BY, the left side may contain grouping/aggregation pushed into the DruidQuery itself. In this case, getDataSource() returns the raw table scan instead of a subquery that wraps the aggregation.

Fix

Added an additional check in toDruidQuery(): when the computed updatedLeftQuery is a GroupByQuery (i.e., contains grouping), wrap it in a QueryDataSource even if computeLeftRequiresSubquery() returns false. This ensures the grouping/aggregation logic is preserved in the UNNEST data source.

Changes

• sql/src/main/java/org/apache/druid/sql/calcite/rel/DruidCorrelateUnnestRel.java: Added instanceof GroupByQuery check in the toDruidQuery() method to detect when the left query contains grouping, and use QueryDataSource accordingly.

[FrankChen021]

Severity	Findings
P0	0
P1	1
P2	0
P3	0
Total	1

Reviewed 1 of 1 changed files.

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This is an automated review by Codex GPT-5.6-Luna(max)

[FrankChen021]

[P1] GroupBy detection is unreachable

This check runs only when the partial-query stage is SCAN, WHERE_FILTER, or SELECT_PROJECT. DruidQuery emits GroupByQuery only when an aggregate exists, which places the stage at AGGREGATE or later. Therefore the new condition cannot change the data-source choice for the reported grouped-left-side case, leaving the fix ineffective.