Add new function planstate_tree_walker.

ExplainPreScanNode knows how to iterate over a generic tree of plan
states; factor that logic out into a separate walker function so that
other code, such as upcoming patches for parallel query, can also use
it.

Patch by me, reviewed by Tom Lane.

src/backend/nodes/nodeFuncs.c

@@ -18,6 +18,7 @@
 #include "catalog/pg_type.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
+#include "nodes/execnodes.h"
 #include "nodes/nodeFuncs.h"
 #include "nodes/relation.h"
 #include "utils/builtins.h"
@@ -26,6 +27,10 @@
 
 static bool expression_returns_set_walker(Node *node, void *context);
 static int	leftmostLoc(int loc1, int loc2);
+static bool planstate_walk_subplans(List *plans, bool (*walker) (),
+												void *context);
+static bool planstate_walk_members(List *plans, PlanState **planstates,
+					   bool (*walker) (), void *context);
 
 
 /*
@@ -3412,3 +3417,123 @@ raw_expression_tree_walker(Node *node,
 	}
 	return false;
 }
+
+/*
+ * planstate_tree_walker --- walk plan state trees
+ *
+ * The walker has already visited the current node, and so we need only
+ * recurse into any sub-nodes it has.
+ */
+bool
+planstate_tree_walker(PlanState *planstate, bool (*walker) (), void *context)
+{
+	Plan	   *plan = planstate->plan;
+
+	/* initPlan-s */
+	if (planstate_walk_subplans(planstate->initPlan, walker, context))
+		return true;
+
+	/* lefttree */
+	if (outerPlanState(planstate))
+	{
+		if (walker(outerPlanState(planstate), context))
+			return true;
+	}
+
+	/* righttree */
+	if (innerPlanState(planstate))
+	{
+		if (walker(innerPlanState(planstate), context))
+			return true;
+	}
+
+	/* special child plans */
+	switch (nodeTag(plan))
+	{
+		case T_ModifyTable:
+			if (planstate_walk_members(((ModifyTable *) plan)->plans,
+								  ((ModifyTableState *) planstate)->mt_plans,
+									   walker, context))
+				return true;
+			break;
+		case T_Append:
+			if (planstate_walk_members(((Append *) plan)->appendplans,
+									((AppendState *) planstate)->appendplans,
+									   walker, context))
+				return true;
+			break;
+		case T_MergeAppend:
+			if (planstate_walk_members(((MergeAppend *) plan)->mergeplans,
+								((MergeAppendState *) planstate)->mergeplans,
+									   walker, context))
+				return true;
+			break;
+		case T_BitmapAnd:
+			if (planstate_walk_members(((BitmapAnd *) plan)->bitmapplans,
+								 ((BitmapAndState *) planstate)->bitmapplans,
+									   walker, context))
+				return true;
+			break;
+		case T_BitmapOr:
+			if (planstate_walk_members(((BitmapOr *) plan)->bitmapplans,
+								  ((BitmapOrState *) planstate)->bitmapplans,
+									   walker, context))
+				return true;
+			break;
+		case T_SubqueryScan:
+			if (walker(((SubqueryScanState *) planstate)->subplan, context))
+				return true;
+			break;
+		default:
+			break;
+	}
+
+	/* subPlan-s */
+	if (planstate_walk_subplans(planstate->subPlan, walker, context))
+		return true;
+
+	return false;
+}
+
+/*
+ * Walk a list of SubPlans (or initPlans, which also use SubPlan nodes).
+ */
+static bool
+planstate_walk_subplans(List *plans, bool (*walker) (), void *context)
+{
+	ListCell   *lc;
+
+	foreach(lc, plans)
+	{
+		SubPlanState *sps = (SubPlanState *) lfirst(lc);
+
+		Assert(IsA(sps, SubPlanState));
+		if (walker(sps->planstate, context))
+			return true;
+	}
+
+	return false;
+}
+
+/*
+ * Walk the constituent plans of a ModifyTable, Append, MergeAppend,
+ * BitmapAnd, or BitmapOr node.
+ *
+ * Note: we don't actually need to examine the Plan list members, but
+ * we need the list in order to determine the length of the PlanState array.
+ */
+static bool
+planstate_walk_members(List *plans, PlanState **planstates,
+					   bool (*walker) (), void *context)
+{
+	int			nplans = list_length(plans);
+	int			j;
+
+	for (j = 0; j < nplans; j++)
+	{
+		if (walker(planstates[j], context))
+			return true;
+	}
+
+	return false;
+}