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Improve planner's handling of SetOp plans.

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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
This commit is contained in:
Tom Lane
2024-12-19 17:02:25 -05:00
parent 2762792952
commit 8d96f57d5c
8 changed files with 365 additions and 332 deletions
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7
src/backend/optimizer/plan/planner.c
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@@ -616,7 +616,7 @@ standard_planner(Query *parse, const char *query_string, int cursorOptions,
* setops is used for set operation subqueries to provide the subquery with
* the context in which it's being used so that Paths correctly sorted for the
* set operation can be generated. NULL when not planning a set operation
* child.
* child, or when a child of a set op that isn't interested in sorted input.
*
* Basically, this routine does the stuff that should only be done once
* per Query object. It then calls grouping_planner. At one time,
@@ -1350,7 +1350,7 @@ preprocess_phv_expression(PlannerInfo *root, Expr *expr)
* setops is used for set operation subqueries to provide the subquery with
* the context in which it's being used so that Paths correctly sorted for the
* set operation can be generated. NULL when not planning a set operation
* child.
* child, or when a child of a set op that isn't interested in sorted input.
*
* Returns nothing; the useful output is in the Paths we attach to the
* (UPPERREL_FINAL, NULL) upperrel in *root. In addition,
@@ -3467,8 +3467,7 @@ standard_qp_callback(PlannerInfo *root, void *extra)
tlist);
/* setting setop_pathkeys might be useful to the union planner */
if (qp_extra->setop != NULL &&
set_operation_ordered_results_useful(qp_extra->setop))
if (qp_extra->setop != NULL)
{
List *groupClauses;
bool sortable;