8000 Improve planner's handling of SetOp plans. · postgres/postgres@8d96f57 · GitHub
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Improve planner's handling of SetOp plans.
Remove the code for inserting flag columns in the inputs of a SetOp. That was the only reason why there would be resjunk columns in a set-operations plan tree, so we can get rid of some code that supported that, too. Get rid of choose_hashed_setop() in favor of building Paths for the hashed and sorted alternatives, and letting them fight it out within add_path(). Remove set_operation_ordered_results_useful(), which was giving wrong answers due to examining the wrong ancestor node: we need to examine the immediate SetOperationStmt parent not the topmost node. Instead make each caller of recurse_set_operations() pass down the relevant parent node. (This thinko seems to have led only to wasted planning cycles and possibly-inferior plans, not wrong query answers. Perhaps we should back-patch it, but I'm not doing so right now.) Teach generate_nonunion_paths() to consider pre-sorted inputs for sorted SetOps, rather than always generating a Sort node. Patch by me; thanks to Richard Guo and David Rowley for review. Discussion: https://postgr.es/m/1850138.1731549611@sss.pgh.pa.us
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‎src/backend/optimizer/plan/planner.c

Lines changed: 3 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -616,7 +616,7 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
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* setops is used for set operation subqueries to provide the subquery with
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* the context in which it's being used so that Paths correctly sorted for the
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* set operation can be generated. NULL when not planning a set operation
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* child.
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* child, or when a child of a set op that isn't interested in sorted input.
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*
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* Basically, this routine does the stuff that should only be done once
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* per Query object. It then calls grouping_planner. At one time,
@@ -1350,7 +1350,7 @@ preprocess_phv_expression(PlannerInfo *root, Expr *expr)
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* setops is used for set operation subqueries to provide the subquery with
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* the context in which it's being used so that Paths correctly sorted for the
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* set operation can be generated. NULL when not planning a set operation
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* child.
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* child, or when a child of a set op that isn't interested in sorted input.
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*
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* Returns nothing; the useful output is in the Paths we attach to the
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* (UPPERREL_FINAL, NULL) upperrel in *root. In addition,
@@ -3467,8 +3467,7 @@ standard_qp_callback(PlannerInfo *root, void *extra)
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tlist);
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/* setting setop_pathkeys might be useful to the union planner */
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if (qp_extra->setop != NULL &&
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set_operation_ordered_results_useful(qp_extra->setop))
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if (qp_extra->setop != NULL)
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{
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List *groupClauses;
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bool sortable;

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