Fix create_unique_plan() so it doesn't generate useless entries in the

output targetlist of the Unique or HashAgg plan.  This code was OK when
written, but subsequent changes to use "physical tlists" where possible
had broken it: given an input subplan that has extra variables added to
avoid a projection step, it would copy those extra variables into the
upper tlist, which is pointless since a projection has to happen anyway.

src/backend/optimizer/plan/createplan.c

@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.193 2005/07/02 23:00:41 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/createplan.c,v 1.194 2005/07/15 22:02:51 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -525,34 +525,33 @@ create_unique_plan(PlannerInfo *root, UniquePath *best_path)
 	Plan	   *plan;
 	Plan	   *subplan;
 	List	   *uniq_exprs;
-	int			numGroupCols;
-	AttrNumber *groupColIdx;
-	int			groupColPos;
 	List	   *newtlist;
 	int			nextresno;
 	bool		newitems;
+	int			numGroupCols;
+	AttrNumber *groupColIdx;
+	int			groupColPos;
 	ListCell   *l;
 
 	subplan = create_plan(root, best_path->subpath);
 
+	/* Done if we don't need to do any actual unique-ifying */
+	if (best_path->umethod == UNIQUE_PATH_NOOP)
+		return subplan;
+
 	/*----------
 	 * As constructed, the subplan has a "flat" tlist containing just the
 	 * Vars needed here and at upper levels.  The values we are supposed
 	 * to unique-ify may be expressions in these variables.  We have to
-	 * add any such expressions to the subplan's tlist.  We then build
-	 * control information showing which subplan output columns are to be
-	 * examined by the grouping step.  (Since we do not remove any
-	 * existing subplan outputs, not all the output columns may be used
-	 * for grouping.)
+	 * add any such expressions to the subplan's tlist.
 	 *
-	 * Note: the reason we don't remove any subplan outputs is that there
-	 * are scenarios where a Var is needed at higher levels even though
-	 * it is not one of the nominal outputs of an IN clause.	Consider
-	 *		WHERE x IN (SELECT y FROM t1,t2 WHERE y = z)
-	 * Implied equality deduction will generate an "x = z" clause, which may
-	 * get used instead of "x = y" in the upper join step.  Therefore the
-	 * sub-select had better deliver both y and z in its targetlist.
-	 * It is sufficient to unique-ify on y, however.
+	 * The subplan may have a "physical" tlist if it is a simple scan plan.
+	 * This should be left as-is if we don't need to add any expressions;
+	 * but if we do have to add expressions, then a projection step will be
+	 * needed at runtime anyway, and so we may as well remove unneeded items.
+	 * Therefore newtlist starts from build_relation_tlist() not just a
+	 * copy of the subplan's tlist; and we don't install it into the subplan
+	 * unless stuff has to be added.
 	 *
 	 * To find the correct list of values to unique-ify, we look in the
 	 * information saved for IN expressions.  If this code is ever used in
@@ -574,12 +573,8 @@ create_unique_plan(PlannerInfo *root, UniquePath *best_path)
 	if (l == NULL)				/* fell out of loop? */
 		elog(ERROR, "could not find UniquePath in in_info_list");
 
-	/* set up to record positions of unique columns */
-	numGroupCols = list_length(uniq_exprs);
-	groupColIdx = (AttrNumber *) palloc(numGroupCols * sizeof(AttrNumber));
-	groupColPos = 0;
-	/* not sure if tlist might be shared with other nodes, so copy */
-	newtlist = copyObject(subplan->targetlist);
+	/* initialize modified subplan tlist as just the "required" vars */
+	newtlist = build_relation_tlist(best_path->path.parent);
 	nextresno = list_length(newtlist) + 1;
 	newitems = false;
 
@@ -599,7 +594,6 @@ create_unique_plan(PlannerInfo *root, UniquePath *best_path)
 			nextresno++;
 			newitems = true;
 		}
-		groupColIdx[groupColPos++] = tle->resno;
 	}
 
 	if (newitems)
@@ -614,9 +608,27 @@ create_unique_plan(PlannerInfo *root, UniquePath *best_path)
 			subplan->targetlist = newtlist;
 	}
 
-	/* Done if we don't need to do any actual unique-ifying */
-	if (best_path->umethod == UNIQUE_PATH_NOOP)
-		return subplan;
+	/*
+	 * Build control information showing which subplan output columns are
+	 * to be examined by the grouping step.  Unfortunately we can't merge this
+	 * with the previous loop, since we didn't then know which version of the
+	 * subplan tlist we'd end up using.
+	 */
+	newtlist = subplan->targetlist;
+	numGroupCols = list_length(uniq_exprs);
+	groupColIdx = (AttrNumber *) palloc(numGroupCols * sizeof(AttrNumber));
+	groupColPos = 0;
+
+	foreach(l, uniq_exprs)
+	{
+		Node	   *uniqexpr = lfirst(l);
+		TargetEntry *tle;
+
+		tle = tlist_member(uniqexpr, newtlist);
+		if (!tle)				/* shouldn't happen */
+			elog(ERROR, "failed to find unique expression in subplan tlist");
+		groupColIdx[groupColPos++] = tle->resno;
+	}
 
 	if (best_path->umethod == UNIQUE_PATH_HASH)
 	{
@@ -624,8 +636,13 @@ create_unique_plan(PlannerInfo *root, UniquePath *best_path)
 
 		numGroups = (long) Min(best_path->rows, (double) LONG_MAX);
 
+		/*
+		 * Since the Agg node is going to project anyway, we can give it
+		 * the minimum output tlist, without any stuff we might have added
+		 * to the subplan tlist.
+		 */
 		plan = (Plan *) make_agg(root,
-								 copyObject(subplan->targetlist),
+								 build_relation_tlist(best_path->path.parent),
 								 NIL,
 								 AGG_HASHED,
 								 numGroupCols,