Make UtilityContainsQuery recurse until it finds a non-utility Query.

The callers of UtilityContainsQuery want it to return a non-utility Query
if it returns anything at all.  However, since we made CREATE TABLE
AS/SELECT INTO into a utility command instead of a variant of SELECT,
a command like "EXPLAIN SELECT INTO" results in two nested utility
statements.  So what we need UtilityContainsQuery to do is drill down
to the bottom non-utility Query.

I had thought of this possibility in setrefs.c, and fixed it there by
looping around the UtilityContainsQuery call; but overlooked that the call
sites in plancache.c have a similar issue.  In those cases it's
notationally inconvenient to provide an external loop, so let's redefine
UtilityContainsQuery as recursing down to a non-utility Query instead.

Noted by Rushabh Lathia.  This is a somewhat cleaned-up version of his
proposed patch.

src/backend/optimizer/plan/setrefs.c

@@ -1937,7 +1937,7 @@ extract_query_dependencies_walker(Node *node, PlannerInfo *context)
 		Query	   *query = (Query *) node;
 		ListCell   *lc;
 
-		while (query->commandType == CMD_UTILITY)
+		if (query->commandType == CMD_UTILITY)
 		{
 			/*
 			 * Ignore utility statements, except those (such as EXPLAIN) that

src/backend/tcop/utility.c

@@ -1351,25 +1351,39 @@ QueryReturnsTuples(Query *parsetree)
  * UtilityContainsQuery
  *		Return the contained Query, or NULL if there is none
  *
- * Certain utility statements, such as EXPLAIN, contain a Query.
+ * Certain utility statements, such as EXPLAIN, contain a plannable Query.
  * This function encapsulates knowledge of exactly which ones do.
  * We assume it is invoked only on already-parse-analyzed statements
  * (else the contained parsetree isn't a Query yet).
+ *
+ * In some cases (currently, only EXPLAIN of CREATE TABLE AS/SELECT INTO),
+ * potentially Query-containing utility statements can be nested.  This
+ * function will drill down to a non-utility Query, or return NULL if none.
  */
 Query *
 UtilityContainsQuery(Node *parsetree)
 {
+	Query	   *qry;
+
 	switch (nodeTag(parsetree))
 	{
 		case T_ExplainStmt:
-			Assert(IsA(((ExplainStmt *) parsetree)->query, Query));
-			return (Query *) ((ExplainStmt *) parsetree)->query;
+			qry = (Query *) ((ExplainStmt *) parsetree)->query;
+			Assert(IsA(qry, Query));
+			if (qry->commandType == CMD_UTILITY)
+				return UtilityContainsQuery(qry->utilityStmt);
+			return qry;
 
 		case T_CreateTableAsStmt:
 			/* might or might not contain a Query ... */
-			if (IsA(((CreateTableAsStmt *) parsetree)->query, Query))
-				return (Query *) ((CreateTableAsStmt *) parsetree)->query;
-			Assert(IsA(((CreateTableAsStmt *) parsetree)->query, ExecuteStmt));
+			qry = (Query *) ((CreateTableAsStmt *) parsetree)->query;
+			if (IsA(qry, Query))
+			{
+				/* Recursion currently can't be necessary here */
+				Assert(qry->commandType != CMD_UTILITY);
+				return qry;
+			}
+			Assert(IsA(qry, ExecuteStmt));
 			return NULL;
 
 		default: