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Commitff72280

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When we have successfully optimized a MIN or MAX aggregate into an indexscan,
the query result must be exactly one row (since we don't do this when there'sany GROUP BY). Therefore any ORDER BY or DISTINCT attached to the query isuseless and can be dropped. Aside from saving useless cycles, this protectsus against problems with matching the hacked-up tlist entries to sort clauses,as seen in a bug report from Taiki Yamaguchi. We might need to work harderif we ever try to optimize grouped queries with this approach, but thissolution will do for now.
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‎src/backend/optimizer/plan/planner.c

Lines changed: 12 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
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*
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*
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* IDENTIFICATION
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* $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.227 2008/03/18 22:04:14 tgl Exp $
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* $PostgreSQL: pgsql/src/backend/optimizer/plan/planner.c,v 1.228 2008/03/27 19:06:14 tgl Exp $
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*
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*-------------------------------------------------------------------------
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*/
@@ -950,6 +950,17 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
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* right tlist, and it has no sort order.
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*/
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current_pathkeys=NIL;
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/*
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* In fact, since we don't optimize grouped aggregates, it
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* needs no sort order --- there must be exactly one output row,
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* and so any ORDER BY or DISTINCT attached to the query is
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* useless and can be dropped. Aside from saving useless cycles,
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* this protects us against problems with matching the hacked-up
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* tlist entries to sort clauses.
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*/
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Assert(!parse->groupClause);
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parse->sortClause=NULL;
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parse->distinctClause=NULL;
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}
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else
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{

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