Fix overoptimistic assumptions in column width estimation for subqueries.

set_append_rel_pathlist supposed that, while computing per-column width
estimates for the appendrel, it could ignore child rels for which the
translated reltargetlist entry wasn't a Var.  This gave rise to completely
silly estimates in some common cases, such as constant outputs from some or
all of the arms of a UNION ALL.  Instead, fall back on get_typavgwidth to
estimate from the value's datatype; which might be a poor estimate but at
least it's not completely wacko.

That problem was exposed by an Assert in set_subquery_size_estimates, which
unfortunately was still overoptimistic even with that fix, since we don't
compute attr_widths estimates for appendrels that are entirely excluded by
constraints.  So remove the Assert; we'll just fall back on get_typavgwidth
in such cases.

Also, since set_subquery_size_estimates calls set_baserel_size_estimates
which calls set_rel_width, there's no need for set_subquery_size_estimates
to call get_typavgwidth; set_rel_width will handle it for us if we just
leave the estimate set to zero.  Remove the unnecessary code.

Per report from Erik Rijkers and subsequent investigation.

src/backend/optimizer/path/allpaths.c

@@ -403,7 +403,15 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 			continue;
 		}
 
-		/* CE failed, so finish copying targetlist and join quals */
+		/*
+		 * CE failed, so finish copying/modifying targetlist and join quals.
+		 *
+		 * Note: the resulting childrel->reltargetlist may contain arbitrary
+		 * expressions, which normally would not occur in a reltargetlist.
+		 * That is okay because nothing outside of this routine will look at
+		 * the child rel's reltargetlist.  We do have to cope with the case
+		 * while constructing attr_widths estimates below, though.
+		 */
 		childrel->joininfo = (List *)
 			adjust_appendrel_attrs((Node *) rel->joininfo,
 								   appinfo);
@@ -486,23 +494,36 @@ set_append_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 			parent_rows += childrel->rows;
 			parent_size += childrel->width * childrel->rows;
 
+			/*
+			 * Accumulate per-column estimates too.  We need not do anything
+			 * for PlaceHolderVars in the parent list.  If child expression
+			 * isn't a Var, or we didn't record a width estimate for it, we
+			 * have to fall back on a datatype-based estimate.
+			 *
+			 * By construction, child's reltargetlist is 1-to-1 with parent's.
+			 */
 			forboth(parentvars, rel->reltargetlist,
 					childvars, childrel->reltargetlist)
 			{
 				Var		   *parentvar = (Var *) lfirst(parentvars);
-				Var		   *childvar = (Var *) lfirst(childvars);
-
-				/*
-				 * Accumulate per-column estimates too.  Whole-row Vars and
-				 * PlaceHolderVars can be ignored here.
-				 */
-				if (IsA(parentvar, Var) &&
-					IsA(childvar, Var))
+				Node	   *childvar = (Node *) lfirst(childvars);
+
+				if (IsA(parentvar, Var))
 				{
 					int			pndx = parentvar->varattno - rel->min_attr;
-					int			cndx = childvar->varattno - childrel->min_attr;
-
-					parent_attrsizes[pndx] += childrel->attr_widths[cndx] * childrel->rows;
+					int32		child_width = 0;
+
+					if (IsA(childvar, Var))
+					{
+						int		cndx = ((Var *) childvar)->varattno - childrel->min_attr;
+
+						child_width = childrel->attr_widths[cndx];
+					}
+					if (child_width <= 0)
+						child_width = get_typavgwidth(exprType(childvar),
+													  exprTypmod(childvar));
+					Assert(child_width > 0);
+					parent_attrsizes[pndx] += child_width * childrel->rows;
 				}
 			}
 		}

src/backend/optimizer/path/costsize.c

@@ -3238,14 +3238,14 @@ set_subquery_size_estimates(PlannerInfo *root, RelOptInfo *rel,
 	/*
 	 * Compute per-output-column width estimates by examining the subquery's
 	 * targetlist.	For any output that is a plain Var, get the width estimate
-	 * that was made while planning the subquery.  Otherwise, fall back on a
-	 * datatype-based estimate.
+	 * that was made while planning the subquery.  Otherwise, we leave it to
+	 * set_rel_width to fill in a datatype-based default estimate.
 	 */
 	foreach(lc, subroot->parse->targetList)
 	{
 		TargetEntry *te = (TargetEntry *) lfirst(lc);
 		Node	   *texpr = (Node *) te->expr;
-		int32		item_width;
+		int32		item_width = 0;
 
 		Assert(IsA(te, TargetEntry));
 		/* junk columns aren't visible to upper query */
@@ -3256,8 +3256,14 @@ set_subquery_size_estimates(PlannerInfo *root, RelOptInfo *rel,
 		 * XXX This currently doesn't work for subqueries containing set
 		 * operations, because the Vars in their tlists are bogus references
 		 * to the first leaf subquery, which wouldn't give the right answer
-		 * even if we could still get to its PlannerInfo.  So fall back on
-		 * datatype in that case.
+		 * even if we could still get to its PlannerInfo.
+		 *
+		 * Also, the subquery could be an appendrel for which all branches are
+		 * known empty due to constraint exclusion, in which case
+		 * set_append_rel_pathlist will have left the attr_widths set to zero.
+		 *
+		 * In either case, we just leave the width estimate zero until
+		 * set_rel_width fixes it.
 		 */
 		if (IsA(texpr, Var) &&
 			subroot->parse->setOperations == NULL)
@@ -3267,11 +3273,6 @@ set_subquery_size_estimates(PlannerInfo *root, RelOptInfo *rel,
 
 			item_width = subrel->attr_widths[var->varattno - subrel->min_attr];
 		}
-		else
-		{
-			item_width = get_typavgwidth(exprType(texpr), exprTypmod(texpr));
-		}
-		Assert(item_width > 0);
 		Assert(te->resno >= rel->min_attr && te->resno <= rel->max_attr);
 		rel->attr_widths[te->resno - rel->min_attr] = item_width;
 	}