Split QTW_EXAMINE_RTES flag into QTW_EXAMINE_RTES_BEFORE/_AFTER.

This change allows callers of query_tree_walker() to choose whether
to visit an RTE before or after visiting the contents of the RTE
(i.e., prefix or postfix tree order).  All existing users of
QTW_EXAMINE_RTES want the QTW_EXAMINE_RTES_BEFORE behavior, but
an upcoming patch will want QTW_EXAMINE_RTES_AFTER, and it seems
like a potentially useful change on its own.

Andreas Karlsson (extracted from CTE inlining patch)

Discussion: https://postgr.es/m/8810.1542402910@sss.pgh.pa.us

src/backend/nodes/nodeFuncs.c

@@ -2255,7 +2255,7 @@ expression_tree_walker(Node *node,
  * Some callers want to suppress visitation of certain items in the sub-Query,
  * typically because they need to process them specially, or don't actually
  * want to recurse into subqueries.  This is supported by the flags argument,
- * which is the bitwise OR of flag values to suppress visitation of
+ * which is the bitwise OR of flag values to add or suppress visitation of
  * indicated items.  (More flag bits may be added as needed.)
  */
 bool
@@ -2314,8 +2314,12 @@ range_table_walker(List *rtable,
 	{
 		RangeTblEntry *rte = (RangeTblEntry *) lfirst(rt);
 
-		/* For historical reasons, visiting RTEs is not the default */
-		if (flags & QTW_EXAMINE_RTES)
+		/*
+		 * Walkers might need to examine the RTE node itself either before or
+		 * after visiting its contents (or, conceivably, both).  Note that if
+		 * you specify neither flag, the walker won't visit the RTE at all.
+		 */
+		if (flags & QTW_EXAMINE_RTES_BEFORE)
 			if (walker(rte, context))
 				return true;
 
@@ -2355,6 +2359,10 @@ range_table_walker(List *rtable,
 
 		if (walker(rte->securityQuals, context))
 			return true;
+
+		if (flags & QTW_EXAMINE_RTES_AFTER)
+			if (walker(rte, context))
+				return true;
 	}
 	return false;
 }

src/backend/optimizer/plan/setrefs.c

@@ -340,7 +340,7 @@ flatten_unplanned_rtes(PlannerGlobal *glob, RangeTblEntry *rte)
 	(void) query_tree_walker(rte->subquery,
 							 flatten_rtes_walker,
 							 (void *) glob,
-							 QTW_EXAMINE_RTES);
+							 QTW_EXAMINE_RTES_BEFORE);
 }
 
 static bool
@@ -363,7 +363,7 @@ flatten_rtes_walker(Node *node, PlannerGlobal *glob)
 		return query_tree_walker((Query *) node,
 								 flatten_rtes_walker,
 								 (void *) glob,
-								 QTW_EXAMINE_RTES);
+								 QTW_EXAMINE_RTES_BEFORE);
 	}
 	return expression_tree_walker(node, flatten_rtes_walker,
 								  (void *) glob);

src/backend/rewrite/rewriteManip.c

@@ -761,7 +761,7 @@ IncrementVarSublevelsUp_walker(Node *node,
 		result = query_tree_walker((Query *) node,
 								   IncrementVarSublevelsUp_walker,
 								   (void *) context,
-								   QTW_EXAMINE_RTES);
+								   QTW_EXAMINE_RTES_BEFORE);
 		context->min_sublevels_up--;
 		return result;
 	}
@@ -785,7 +785,7 @@ IncrementVarSublevelsUp(Node *node, int delta_sublevels_up,
 	query_or_expression_tree_walker(node,
 									IncrementVarSublevelsUp_walker,
 									(void *) &context,
-									QTW_EXAMINE_RTES);
+									QTW_EXAMINE_RTES_BEFORE);
 }
 
 /*
@@ -804,7 +804,7 @@ IncrementVarSublevelsUp_rtable(List *rtable, int delta_sublevels_up,
 	range_table_walker(rtable,
 					   IncrementVarSublevelsUp_walker,
 					   (void *) &context,
-					   QTW_EXAMINE_RTES);
+					   QTW_EXAMINE_RTES_BEFORE);
 }
 
 

src/include/nodes/nodeFuncs.h

@@ -22,8 +22,11 @@
 #define QTW_IGNORE_RC_SUBQUERIES	0x03	/* both of above */
 #define QTW_IGNORE_JOINALIASES		0x04	/* JOIN alias var lists */
 #define QTW_IGNORE_RANGE_TABLE		0x08	/* skip rangetable entirely */
-#define QTW_EXAMINE_RTES			0x10	/* examine RTEs */
-#define QTW_DONT_COPY_QUERY			0x20	/* do not copy top Query */
+#define QTW_EXAMINE_RTES_BEFORE		0x10	/* examine RTE nodes before their
+											 * contents */
+#define QTW_EXAMINE_RTES_AFTER		0x20	/* examine RTE nodes after their
+											 * contents */
+#define QTW_DONT_COPY_QUERY			0x40	/* do not copy top Query */
 
 /* callback function for check_functions_in_node */
 typedef bool (*check_function_callback) (Oid func_id, void *context);