Fix estimates for ModifyTable paths without RETURNING.

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

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index c1fc866cbf911049639c83be78b560bdc9b5a8fc..5281a2f9983ffcd7c6706a1f3778c1f41bdd034b 100644 (file)
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -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;