Fix mis-planning of repeated application of a projection.

tglsfdc · tglsfdc · commit fe194f731fbf · 2021-05-31T12:03:00.000-04:00
create_projection_plan contains a hidden assumption (here made explicit by an Assert) that a projection-capable Path will yield a projection-capable Plan. Unfortunately, that assumption is violated only a few lines away, by create_projection_plan itself. This means that two stacked ProjectionPaths can yield an outcome where we try to jam the upper path's tlist into a non-projection-capable child node, resulting in an invalid plan. There isn't any good reason to have stacked ProjectionPaths; indeed the whole concept is faulty, since the set of Vars/Aggs/etc needed by the upper one wouldn't necessarily be available in the output of the lower one, nor could the lower one create such values if they weren't available from its input. Hence, we can fix this by adjusting create_projection_path to strip any top-level ProjectionPath from the subpath it's given. (This amounts to saying "oh, we changed our minds about what we need to project here".) The test case added here only fails in v13 and HEAD; before that, we don't attempt to shove the Sort into the parallel part of the plan, for reasons that aren't entirely clear to me. However, all the directly-related code looks generally the same as far back as v11, where the hazard was introduced (by d7c19e6). So I've got no faith that the same type of bug doesn't exist in v11 and v12, given the right test case. Hence, back-patch the code changes, but not the irrelevant test case, into those branches. Per report from Bas Poot. Discussion: https://postgr.es/m/534fca83789c4a378c7de379e9067d4f@politie.nl
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
@@ -1671,6 +1671,7 @@ create_projection_plan(PlannerInfo *root, ProjectionPath *best_path, int flags)
 		 */
 		subplan = create_plan_recurse(root, best_path->subpath,
 									  CP_IGNORE_TLIST);
+		Assert(is_projection_capable_plan(subplan));
 		tlist = build_path_tlist(root, &best_path->path);
 	}
 	else
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
@@ -2440,7 +2440,23 @@ create_projection_path(PlannerInfo *root,
 					   PathTarget *target)
 {
 	ProjectionPath *pathnode = makeNode(ProjectionPath);
-	PathTarget *oldtarget = subpath->pathtarget;
+	PathTarget *oldtarget;
+
+	/*
+	 * We mustn't put a ProjectionPath directly above another; it's useless
+	 * and will confuse create_projection_plan.  Rather than making sure all
+	 * callers handle that, let's implement it here, by stripping off any
+	 * ProjectionPath in what we're given.  Given this rule, there won't be
+	 * more than one.
+	 */
+	if (IsA(subpath, ProjectionPath))
+	{
+		ProjectionPath *subpp = (ProjectionPath *) subpath;
+
+		Assert(subpp->path.parent == rel);
+		subpath = subpp->subpath;
+		Assert(!IsA(subpath, ProjectionPath));
+	}
 
 	pathnode->path.pathtype = T_Result;
 	pathnode->path.parent = rel;
@@ -2466,6 +2482,7 @@ create_projection_path(PlannerInfo *root,
 	 * Note: in the latter case, create_projection_plan has to recheck our
 	 * conclusion; see comments therein.
 	 */
+	oldtarget = subpath->pathtarget;
 	if (is_projection_capable_path(subpath) ||
 		equal(oldtarget->exprs, target->exprs))
 	{

Original file line number	Diff line number	Diff line change
`@@ -1671,6 +1671,7 @@ create_projection_plan(PlannerInfo root, ProjectionPath best_path, int flags)`
`1671`	`1671`	`*/`
`1672`	`1672`	`subplan = create_plan_recurse(root, best_path->subpath,`
`1673`	`1673`	`CP_IGNORE_TLIST);`
	`1674`	`+ Assert(is_projection_capable_plan(subplan));`
`1674`	`1675`	`tlist = build_path_tlist(root, &best_path->path);`
`1675`	`1676`	`}`
`1676`	`1677`	`else`