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Commitf0f13a3

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Fix estimates for ModifyTable paths without RETURNING.
In the past, we always estimated that a ModifyTable node would emit thesame number of rows as its subpaths. Without a RETURNING clause, thecorrect estimate is zero. Fix, in preparation for a proposed parallelwrite patch that is sensitive to that number.A remaining problem is that for RETURNING queries, the estimated widthis based on subpath output rather than the RETURNING tlist.Reviewed-by: Greg Nancarrow <gregn4422@gmail.com>Discussion:https://postgr.es/m/CAJcOf-cXnB5cnMKqWEp2E2z7Mvcd04iLVmV%3DqpFJrR3AcrTS3g%40mail.gmail.com
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‎src/backend/optimizer/util/pathnode.c

Lines changed: 8 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -3583,15 +3583,18 @@ create_modifytable_path(PlannerInfo *root, RelOptInfo *rel,
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if (lc==list_head(subpaths))/* first node? */
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pathnode->path.startup_cost=subpath->startup_cost;
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pathnode->path.total_cost+=subpath->total_cost;
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pathnode->path.rows+=subpath->rows;
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total_size+=subpath->pathtarget->width*subpath->rows;
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if (returningLists!=NIL)
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{
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pathnode->path.rows+=subpath->rows;
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total_size+=subpath->pathtarget->width*subpath->rows;
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}
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}
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/*
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* Set width to the average width of the subpath outputs. XXX this is
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* totally wrong: we shouldreport zero if noRETURNING, else an average
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*of the RETURNING tlistwidths. But it's what happened historically,
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*and improving it is a taskfor another day.
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* totally wrong: we shouldreturn an average of theRETURNING tlist
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* widths. But it's what happened historically, and improving it is a task
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* for another day.
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*/
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if (pathnode->path.rows>0)
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total_size /=pathnode->path.rows;

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