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Fix estimates for ModifyTable paths without RETURNING.

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In the past, we always estimated that a ModifyTable node would emit the
same number of rows as its subpaths.  Without a RETURNING clause, the
correct estimate is zero.  Fix, in preparation for a proposed parallel
write patch that is sensitive to that number.

A remaining problem is that for RETURNING queries, the estimated width
is 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
This commit is contained in:
Thomas Munro 2020-10-12 20:41:16 +13:00
parent 3fb676504d
commit f0f13a3a08
1 changed files with 8 additions and 5 deletions
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13
src/backend/optimizer/util/pathnode.c
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@ -3583,15 +3583,18 @@ create_modifytable_path(PlannerInfo *root, RelOptInfo *rel,
if (lc == list_head(subpaths)) /* first node? */
pathnode->path.startup_cost = subpath->startup_cost;
pathnode->path.total_cost += subpath->total_cost;
pathnode->path.rows += subpath->rows;
total_size += subpath->pathtarget->width * subpath->rows;
if (returningLists != NIL)
{
pathnode->path.rows += subpath->rows;
total_size += subpath->pathtarget->width * subpath->rows;
}
}
/*
* Set width to the average width of the subpath outputs. XXX this is
* totally wrong: we should report zero if no RETURNING, else an average
* of the RETURNING tlist widths. But it's what happened historically,
* and improving it is a task for another day.
* totally wrong: we should return an average of the RETURNING tlist
* widths. But it's what happened historically, and improving it is a task
* for another day.
*/
if (pathnode->path.rows > 0)
total_size /= pathnode->path.rows;