branch-4.0: [opt](maxcompute) Optimize split generation for LIMIT queries with partition equality predicates #60895#60973
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…rtition equality predicates (#60895) ### What problem does this PR solve? When a MaxCompute query contains only partition equality predicates and a LIMIT clause, use row_offset split strategy to read only the required number of rows instead of generating splits for all data. This reduces split count from potentially many to exactly one, improving query latency for common LIMIT patterns like `SELECT * FROM t WHERE pt='x' LIMIT N`. Key changes: - Add `checkOnlyPartitionEqualityPredicate()` to detect eligible queries - Add `getSplitsWithLimitOptimization()` using SplitByRowOffset with crossPartition=false, reading min(limit, totalRowCount) rows - Add session variable `enable_mc_limit_split_optimization` (default off) - Add timing logs for split generation phases to aid performance diagnosis - Add unit tests for predicate check and limit optimization logic - Add regression tests covering single/multi-partition tables, JOINs, aggregations, subqueries, window functions, and edge cases
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Cherry-picked from #60895