Keep track of what RTIs a Result node is scanning.

Result nodes now include an RTI set, which is only non-NULL when they
have no subplan, and is taken from the relid set of the RelOptInfo that
the Result is generating. ExplainPreScanNode now takes notice of these
RTIs, which means that a few things get schema-qualified in the
regression tests that previously did not. This makes the output more
consistent between cases where some part of the plan tree is replaced by
a Result node and those where this does not happen.

Likewise, pg_overexplain's EXPLAIN (RANGE_TABLE) now displays the RTIs
stored in a Result node just as it already does for other RTI-bearing
node types.

Result nodes also now include a result_reason, which tells us something
about why the Result node was inserted.  Using that information, EXPLAIN
now emits, where relevant, a "Replaces" line describing the origin of
a Result node.

The purpose of these changes is to allow code that inspects a Plan
tree to understand the origin of Result nodes that appear therein.

Discussion: http://postgr.es/m/CA+TgmoYeUZePZWLsSO+1FAN7UPePT_RMEZBKkqYBJVCF1s60=w@mail.gmail.com
Reviewed-by: Alexandra Wang <alexandra.wang.oss@gmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Junwang Zhao <zhjwpku@gmail.com>

src/backend/commands/explain.c

@@ -147,6 +147,7 @@ static void show_buffer_usage(ExplainState *es, const BufferUsage *usage);
 static void show_wal_usage(ExplainState *es, const WalUsage *usage);
 static void show_memory_counters(ExplainState *es,
 								 const MemoryContextCounters *mem_counters);
+static void show_result_replacement_info(Result *result, ExplainState *es);
 static void ExplainIndexScanDetails(Oid indexid, ScanDirection indexorderdir,
 									ExplainState *es);
 static void ExplainScanTarget(Scan *plan, ExplainState *es);
@@ -1229,6 +1230,10 @@ ExplainPreScanNode(PlanState *planstate, Bitmapset **rels_used)
 			*rels_used = bms_add_members(*rels_used,
 										 ((MergeAppend *) plan)->apprelids);
 			break;
+		case T_Result:
+			*rels_used = bms_add_members(*rels_used,
+										 ((Result *) plan)->relids);
+			break;
 		default:
 			break;
 	}
@@ -2232,6 +2237,7 @@ ExplainNode(PlanState *planstate, List *ancestors,
 								   ancestors, es);
 			break;
 		case T_Result:
+			show_result_replacement_info(castNode(Result, plan), es);
 			show_upper_qual((List *) ((Result *) plan)->resconstantqual,
 							"One-Time Filter", planstate, ancestors, es);
 			show_upper_qual(plan->qual, "Filter", planstate, ancestors, es);
@@ -4750,6 +4756,102 @@ show_modifytable_info(ModifyTableState *mtstate, List *ancestors,
 		ExplainCloseGroup("Target Tables", "Target Tables", false, es);
 }
 
+/*
+ * Explain what a "Result" node replaced.
+ */
+static void
+show_result_replacement_info(Result *result, ExplainState *es)
+{
+	StringInfoData buf;
+	int			nrels = 0;
+	int			rti = -1;
+	bool		found_non_result = false;
+	char	   *replacement_type = "???";
+
+	/* If the Result node has a subplan, it didn't replace anything. */
+	if (result->plan.lefttree != NULL)
+		return;
+
+	/* Gating result nodes should have a subplan, and we don't. */
+	Assert(result->result_type != RESULT_TYPE_GATING);
+
+	switch (result->result_type)
+	{
+		case RESULT_TYPE_GATING:
+			replacement_type = "Gating";
+			break;
+		case RESULT_TYPE_SCAN:
+			replacement_type = "Scan";
+			break;
+		case RESULT_TYPE_JOIN:
+			replacement_type = "Join";
+			break;
+		case RESULT_TYPE_UPPER:
+			/* a small white lie */
+			replacement_type = "Aggregate";
+			break;
+		case RESULT_TYPE_MINMAX:
+			replacement_type = "MinMaxAggregate";
+			break;
+	}
+
+	/*
+	 * Build up a comma-separated list of user-facing names for the range
+	 * table entries in the relids set.
+	 */
+	initStringInfo(&buf);
+	while ((rti = bms_next_member(result->relids, rti)) >= 0)
+	{
+		RangeTblEntry *rte = rt_fetch(rti, es->rtable);
+		char	   *refname;
+
+		/*
+		 * add_outer_joins_to_relids will add join RTIs to the relids set of a
+		 * join; if that join is then replaced with a Result node, we may see
+		 * such RTIs here. But we want to completely ignore those here,
+		 * because "a LEFT JOIN b ON whatever" is a join between a and b, not
+		 * a join between a, b, and an unnamed join.
+		 */
+		if (rte->rtekind == RTE_JOIN)
+			continue;
+
+		/* Count the number of rels that aren't ignored completely. */
+		++nrels;
+
+		/* Work out what reference name to use and add it to the string. */
+		refname = (char *) list_nth(es->rtable_names, rti - 1);
+		if (refname == NULL)
+			refname = rte->eref->aliasname;
+		if (buf.len > 0)
+			appendStringInfoString(&buf, ", ");
+		appendStringInfoString(&buf, refname);
+
+		/* Keep track of whether we see anything other than RTE_RESULT. */
+		if (rte->rtekind != RTE_RESULT)
+			found_non_result = true;
+	}
+
+	/*
+	 * If this Result node is because of a single RTE that is RTE_RESULT, it
+	 * is not really replacing anything at all, because there's no other
+	 * method for implementing a scan of such an RTE, so we don't display the
+	 * Replaces line in such cases.
+	 */
+	if (nrels <= 1 && !found_non_result &&
+		result->result_type == RESULT_TYPE_SCAN)
+		return;
+
+	/* Say what we replaced, with list of rels if available. */
+	if (buf.len == 0)
+		ExplainPropertyText("Replaces", replacement_type, es);
+	else
+	{
+		char	   *s = psprintf("%s on %s", replacement_type, buf.data);
+
+		ExplainPropertyText("Replaces", s, es);
+	}
+}
+
 /*
  * Explain the constituent plans of an Append, MergeAppend,
  * BitmapAnd, or BitmapOr node.

src/backend/optimizer/plan/createplan.c

@@ -99,7 +99,8 @@ static Gather *create_gather_plan(PlannerInfo *root, GatherPath *best_path);
 static Plan *create_projection_plan(PlannerInfo *root,
 									ProjectionPath *best_path,
 									int flags);
-static Plan *inject_projection_plan(Plan *subplan, List *tlist, bool parallel_safe);
+static Plan *inject_projection_plan(Plan *subplan, List *tlist,
+									bool parallel_safe);
 static Sort *create_sort_plan(PlannerInfo *root, SortPath *best_path, int flags);
 static IncrementalSort *create_incrementalsort_plan(PlannerInfo *root,
 													IncrementalSortPath *best_path, int flags);
@@ -302,7 +303,10 @@ static SetOp *make_setop(SetOpCmd cmd, SetOpStrategy strategy,
 						 List *tlist, Plan *lefttree, Plan *righttree,
 						 List *groupList, long numGroups);
 static LockRows *make_lockrows(Plan *lefttree, List *rowMarks, int epqParam);
-static Result *make_result(List *tlist, Node *resconstantqual, Plan *subplan);
+static Result *make_gating_result(List *tlist, Node *resconstantqual,
+								  Plan *subplan);
+static Result *make_one_row_result(List *tlist, Node *resconstantqual,
+								   RelOptInfo *rel);
 static ProjectSet *make_project_set(List *tlist, Plan *subplan);
 static ModifyTable *make_modifytable(PlannerInfo *root, Plan *subplan,
 									 CmdType operation, bool canSetTag,
@@ -1012,35 +1016,35 @@ static Plan *
 create_gating_plan(PlannerInfo *root, Path *path, Plan *plan,
 				   List *gating_quals)
 {
-	Plan	   *gplan;
-	Plan	   *splan;
+	Result	   *gplan;
 
 	Assert(gating_quals);
 
-	/*
-	 * We might have a trivial Result plan already.  Stacking one Result atop
-	 * another is silly, so if that applies, just discard the input plan.
-	 * (We're assuming its targetlist is uninteresting; it should be either
-	 * the same as the result of build_path_tlist, or a simplified version.)
-	 */
-	splan = plan;
-	if (IsA(plan, Result))
-	{
-		Result	   *rplan = (Result *) plan;
-
-		if (rplan->plan.lefttree == NULL &&
-			rplan->resconstantqual == NULL)
-			splan = NULL;
-	}
-
 	/*
 	 * Since we need a Result node anyway, always return the path's requested
 	 * tlist; that's never a wrong choice, even if the parent node didn't ask
 	 * for CP_EXACT_TLIST.
 	 */
-	gplan = (Plan *) make_result(build_path_tlist(root, path),
-								 (Node *) gating_quals,
-								 splan);
+	gplan = make_gating_result(build_path_tlist(root, path),
+							   (Node *) gating_quals, plan);
+
+	/*
+	 * We might have had a trivial Result plan already.  Stacking one Result
+	 * atop another is silly, so if that applies, just discard the input plan.
+	 * (We're assuming its targetlist is uninteresting; it should be either
+	 * the same as the result of build_path_tlist, or a simplified version.
+	 * However, we preserve the set of relids that it purports to scan and
+	 * attribute that to our replacement Result instead, and likewise for the
+	 * result_type.)
+	 */
+	if (IsA(plan, Result))
+	{
+		Result	   *rplan = (Result *) plan;
+
+		gplan->plan.lefttree = NULL;
+		gplan->relids = rplan->relids;
+		gplan->result_type = rplan->result_type;
+	}
 
 	/*
 	 * Notice that we don't change cost or size estimates when doing gating.
@@ -1054,12 +1058,12 @@ create_gating_plan(PlannerInfo *root, Path *path, Plan *plan,
 	 * in most cases we have only a very bad idea of the probability of the
 	 * gating qual being true.
 	 */
-	copy_plan_costsize(gplan, plan);
+	copy_plan_costsize(&gplan->plan, plan);
 
 	/* Gating quals could be unsafe, so better use the Path's safety flag */
-	gplan->parallel_safe = path->parallel_safe;
+	gplan->plan.parallel_safe = path->parallel_safe;
 
-	return gplan;
+	return &gplan->plan;
 }
 
 /*
@@ -1235,10 +1239,10 @@ create_append_plan(PlannerInfo *root, AppendPath *best_path, int flags)
 		/* Generate a Result plan with constant-FALSE gating qual */
 		Plan	   *plan;
 
-		plan = (Plan *) make_result(tlist,
-									(Node *) list_make1(makeBoolConst(false,
-																	  false)),
-									NULL);
+		plan = (Plan *) make_one_row_result(tlist,
+											(Node *) list_make1(makeBoolConst(false,
+																			  false)),
+											best_path->path.parent);
 
 		copy_generic_path_info(plan, (Path *) best_path);
 
@@ -1636,7 +1640,7 @@ create_group_result_plan(PlannerInfo *root, GroupResultPath *best_path)
 	/* best_path->quals is just bare clauses */
 	quals = order_qual_clauses(root, best_path->quals);
 
-	plan = make_result(tlist, (Node *) quals, NULL);
+	plan = make_one_row_result(tlist, (Node *) quals, best_path->path.parent);
 
 	copy_generic_path_info(&plan->plan, (Path *) best_path);
 
@@ -1933,8 +1937,7 @@ create_projection_plan(PlannerInfo *root, ProjectionPath *best_path, int flags)
 	}
 	else
 	{
-		/* We need a Result node */
-		plan = (Plan *) make_result(tlist, NULL, subplan);
+		plan = (Plan *) make_gating_result(tlist, NULL, subplan);
 
 		copy_generic_path_info(plan, (Path *) best_path);
 	}
@@ -1958,7 +1961,7 @@ inject_projection_plan(Plan *subplan, List *tlist, bool parallel_safe)
 {
 	Plan	   *plan;
 
-	plan = (Plan *) make_result(tlist, NULL, subplan);
+	plan = (Plan *) make_gating_result(tlist, NULL, subplan);
 
 	/*
 	 * In principle, we should charge tlist eval cost plus cpu_per_tuple per
@@ -2436,7 +2439,9 @@ create_minmaxagg_plan(PlannerInfo *root, MinMaxAggPath *best_path)
 	/* Generate the output plan --- basically just a Result */
 	tlist = build_path_tlist(root, &best_path->path);
 
-	plan = make_result(tlist, (Node *) best_path->quals, NULL);
+	plan = make_one_row_result(tlist, (Node *) best_path->quals,
+							   best_path->path.parent);
+	plan->result_type = RESULT_TYPE_MINMAX;
 
 	copy_generic_path_info(&plan->plan, (Path *) best_path);
 
@@ -3887,7 +3892,8 @@ create_resultscan_plan(PlannerInfo *root, Path *best_path,
 			replace_nestloop_params(root, (Node *) scan_clauses);
 	}
 
-	scan_plan = make_result(tlist, (Node *) scan_clauses, NULL);
+	scan_plan = make_one_row_result(tlist, (Node *) scan_clauses,
+									best_path->parent);
 
 	copy_generic_path_info(&scan_plan->plan, best_path);
 
@@ -6922,22 +6928,57 @@ make_limit(Plan *lefttree, Node *limitOffset, Node *limitCount,
 }
 
 /*
- * make_result
- *	  Build a Result plan node
+ * make_gating_result
+ *	  Build a Result plan node that performs projection of a subplan, and/or
+ *	  applies a one time filter (resconstantqual)
  */
 static Result *
-make_result(List *tlist,
-			Node *resconstantqual,
-			Plan *subplan)
+make_gating_result(List *tlist,
+				   Node *resconstantqual,
+				   Plan *subplan)
+{
+	Result	   *node = makeNode(Result);
+	Plan	   *plan = &node->plan;
+
+	Assert(subplan != NULL);
+
+	plan->targetlist = tlist;
+	plan->qual = NIL;
+	plan->lefttree = subplan;
+	plan->righttree = NULL;
+	node->result_type = RESULT_TYPE_GATING;
+	node->resconstantqual = resconstantqual;
+	node->relids = NULL;
+
+	return node;
+}
+
+/*
+ * make_one_row_result
+ *	  Build a Result plan node that returns a single row (or possibly no rows,
+ *	  if the one-time filtered defined by resconstantqual returns false)
+ *
+ * 'rel' should be this path's RelOptInfo. In essence, we're saying that this
+ * Result node generates all the tuples for that RelOptInfo. Note that the same
+ * consideration can never arise in make_gating_result(), because in that case
+ * the tuples are always coming from some subordinate node.
+ */
+static Result *
+make_one_row_result(List *tlist,
+					Node *resconstantqual,
+					RelOptInfo *rel)
 {
 	Result	   *node = makeNode(Result);
 	Plan	   *plan = &node->plan;
 
 	plan->targetlist = tlist;
 	plan->qual = NIL;
-	plan->lefttree = subplan;
+	plan->lefttree = NULL;
 	plan->righttree = NULL;
+	node->result_type = IS_UPPER_REL(rel) ? RESULT_TYPE_UPPER :
+		IS_JOIN_REL(rel) ? RESULT_TYPE_JOIN : RESULT_TYPE_SCAN;
 	node->resconstantqual = resconstantqual;
+	node->relids = rel->relids;
 
 	return node;
 }

src/backend/optimizer/plan/setrefs.c

@@ -1056,6 +1056,8 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset)
 				/* resconstantqual can't contain any subplan variable refs */
 				splan->resconstantqual =
 					fix_scan_expr(root, splan->resconstantqual, rtoffset, 1);
+				/* adjust the relids set */
+				splan->relids = offset_relid_set(splan->relids, rtoffset);
 			}
 			break;
 		case T_ProjectSet: