Add MultiDistinctToCrossJoin optimizer rule for parallel distinct aggregates#20940

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Dandandan:multi-distinct-to-union

Commits on Mar 14, 2026

feat: Add MultiDistinctToUnion optimizer rule for parallel distinct aggregates

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Restore clickbench_extended.slt

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Revert OptimizerContext changes; use existing function_registry trait method

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Update sqllogictest EXPLAIN plans for MultiDistinctToUnion rule

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Switch from UNION ALL to cross join for better memory locality

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cargo fmt

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Rename multi_distinct_to_union.rs to multi_distinct_to_cross_join.rs

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Simplify: eliminate double-match and avoid Expr cloning

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Remove assert_optimized_plan_equal wrapper macro

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Fmt
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