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Richard Guo
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Pathify RHS unique-ification for semijoin planning
There are two implementation techniques for semijoins: one uses the JOIN_SEMI jointype, where the executor emits at most one matching row per left-hand side (LHS) row; the other unique-ifies the right-hand side (RHS) and then performs a plain inner join. The latter technique currently has some drawbacks related to the unique-ification step. * Only the cheapest-total path of the RHS is considered during unique-ification. This may cause us to miss some optimization opportunities; for example, a path with a better sort order might be overlooked simply because it is not the cheapest in total cost. Such a path could help avoid a sort at a higher level, potentially resulting in a cheaper overall plan. * We currently rely on heuristics to choose between hash-based and sort-based unique-ification. A better approach would be to generate paths for both methods and allow add_path() to decide which one is preferable, consistent with how path selection is handled elsewhere in the planner. * In the sort-based implementation, we currently pay no attention to the pathkeys of the input subpath or the resulting output. This can result in redundant sort nodes being added to the final plan. This patch improves semijoin planning by creating a new RelOptInfo for the RHS rel to represent its unique-ified version. It then generates multiple paths that represent elimination of distinct rows from the RHS, considering both a hash-based implementation using the cheapest total path of the original RHS rel, and sort-based implementations that either exploit presorted input paths or explicitly sort the cheapest total path. All resulting paths compete in add_path(), and those deemed worthy of consideration are added to the new RelOptInfo. Finally, the unique-ified rel is joined with the other side of the semijoin using a plain inner join. As a side effect, most of the code related to the JOIN_UNIQUE_OUTER and JOIN_UNIQUE_INNER jointypes -- used to indicate that the LHS or RHS path should be made unique -- has been removed. Besides, the T_Unique path now has the same meaning for both semijoins and upper DISTINCT clauses: it represents adjacent-duplicate removal on presorted input. This patch unifies their handling by sharing the same data structures and functions. This patch also removes the UNIQUE_PATH_NOOP related code along the way, as it is dead code -- if the RHS rel is provably unique, the semijoin should have already been simplified to a plain inner join by analyzejoins.c. Author: Richard Guo <guofenglinux@gmail.com> Reviewed-by: Alexandra Wang <alexandra.wang.oss@gmail.com> Reviewed-by: wenhui qiu <qiuwenhuifx@gmail.com> Discussion: https://postgr.es/m/CAMbWs4-EBnaRvEs7frTLbsXiweSTUXifsteF-d3rvv01FKO86w@mail.gmail.com
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-971
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‎src/backend/optimizer/README

Lines changed: 1 addition & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -640,15 +640,14 @@ RelOptInfo - a relation or joined relations
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GroupResultPath - childless Result plan node (used for degenerate grouping)
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MaterialPath - a Material plan node
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MemoizePath - a Memoize plan node for caching tuples from sub-paths
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UniquePath - remove duplicate rows (either by hashing or sorting)
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GatherPath - collect the results of parallel workers
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GatherMergePath - collect parallel results, preserving their common sort order
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ProjectionPath - a Result plan node with child (used for projection)
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ProjectSetPath - a ProjectSet plan node applied to some sub-path
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SortPath - a Sort plan node applied to some sub-path
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IncrementalSortPath - an IncrementalSort plan node applied to some sub-path
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GroupPath - a Group plan node applied to some sub-path
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UpperUniquePath - a Unique plan node applied to some sub-path
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UniquePath - a Unique plan node applied to some sub-path
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AggPath - an Agg plan node applied to some sub-path
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GroupingSetsPath - an Agg plan node used to implement GROUPING SETS
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MinMaxAggPath - a Result plan node with subplans performing MIN/MAX

‎src/backend/optimizer/path/costsize.c

Lines changed: 7 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -3966,10 +3966,12 @@ final_cost_mergejoin(PlannerInfo *root, MergePath *path,
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* when we should not. Can we do better without expensive selectivity
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* computations?
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*
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* The whole issue is moot if we are working from a unique-ified outer
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* input, or if we know we don't need to mark/restore at all.
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* The whole issue is moot if we know we don't need to mark/restore at
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* all, or if we are working from a unique-ified outer input.
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*/
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if (IsA(outer_path, UniquePath) || path->skip_mark_restore)
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if (path->skip_mark_restore ||
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RELATION_WAS_MADE_UNIQUE(outer_path->parent, extra->sjinfo,
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path->jpath.jointype))
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rescannedtuples = 0;
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else
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{
@@ -4364,7 +4366,8 @@ final_cost_hashjoin(PlannerInfo *root, HashPath *path,
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* because we avoid contaminating the cache with a value that's wrong for
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* non-unique-ified paths.
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*/
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if (IsA(inner_path, UniquePath))
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if (RELATION_WAS_MADE_UNIQUE(inner_path->parent, extra->sjinfo,
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path->jpath.jointype))
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{
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innerbucketsize = 1.0 / virtualbuckets;
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innermcvfreq = 0.0;

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