branch-4.0: [fix](agg) Fix incorrect aggregate merge with duplicate aliases #65025#65056
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branch-4.0: [fix](agg) Fix incorrect aggregate merge with duplicate aliases #65025#65056github-actions[bot] wants to merge 1 commit into
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Related PR: #31811 Problem Summary: A nested aggregate query could return incorrect results when multiple outer group-by aliases referenced the same inner grouping expression. For example, after resolving aliases, the inner grouping keys were `(a, b)` while the outer grouping keys became `(a, a)`. `MergeAggregate` determined whether the groupings were identical by comparing their list sizes. Since both lists contained two elements, it incorrectly considered them equivalent. This allowed `SUM(COUNT(DISTINCT c))` to be merged into `COUNT(DISTINCT c)` while removing grouping key `b`. Values repeated across different `b` groups were consequently counted only once, producing an undercount. This change compares the unique grouping-expression sets after projection replacement. Aggregate layers containing `DISTINCT` are now merged only when their grouping semantics are actually identical. ### Release note Fix incorrect query results for nested aggregates with duplicate group-by aliases and DISTINCT aggregate functions.
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Cherry-picked from #65025