Avoid coercing a whole-row variable that is already coerced.

Marginal efficiency and beautification hack.  I'm not sure whether
this case ever arises currently, but the pending patch for update
tuple routing will cause it to arise.

Amit Khandekar

Discussion: http://postgr.es/m/CAJ3gD9cazfppe7-wwUbabPcQ4_0SubkiPFD1+0r5_DkVNWo5yg@mail.gmail.com

src/backend/rewrite/rewriteManip.c

@@ -1224,6 +1224,7 @@ typedef struct
 	/* Target type when converting whole-row vars */
 	Oid			to_rowtype;
 	bool	   *found_whole_row;	/* output flag */
+	bool		coerced_var;	/* var is under ConvertRowTypeExpr */
 } map_variable_attnos_context;
 
 static Node *
@@ -1267,14 +1268,20 @@ map_variable_attnos_mutator(Node *node,
 					/* Don't convert unless necessary. */
 					if (context->to_rowtype != var->vartype)
 					{
-						ConvertRowtypeExpr *r;
-
 						/* Var itself is converted to the requested type. */
 						newvar->vartype = context->to_rowtype;
 
 						/*
-						 * And a conversion node on top to convert back to the
-						 * original type.
+						 * If this var is already under a ConvertRowtypeExpr,
+						 * we don't have to add another one.
+						 */
+						if (!context->coerced_var)
+						{
+							ConvertRowtypeExpr *r;
+
+							/*
+							 * And a conversion node on top to convert back to
+							 * the original type.
 							 */
 							r = makeNode(ConvertRowtypeExpr);
 							r->arg = (Expr *) newvar;
@@ -1286,10 +1293,33 @@ map_variable_attnos_mutator(Node *node,
 						}
 					}
 				}
+			}
 			return (Node *) newvar;
 		}
 		/* otherwise fall through to copy the var normally */
 	}
+	else if (IsA(node, ConvertRowtypeExpr))
+	{
+		ConvertRowtypeExpr *r = (ConvertRowtypeExpr *) node;
+
+		/*
+		 * If this is coercing a var (which is typical), convert only the var,
+		 * as against adding another ConvertRowtypeExpr over it.
+		 */
+		if (IsA(r->arg, Var))
+		{
+			ConvertRowtypeExpr *newnode;
+
+			newnode = (ConvertRowtypeExpr *) palloc(sizeof(ConvertRowtypeExpr));
+			*newnode = *r;
+			context->coerced_var = true;
+			newnode->arg = (Expr *) map_variable_attnos_mutator((Node *) r->arg, context);
+			context->coerced_var = false;
+
+			return (Node *) newnode;
+		}
+		/* Else fall through the expression tree mutator */
+	}
 	else if (IsA(node, Query))
 	{
 		/* Recurse into RTE subquery or not-yet-planned sublink subquery */
@@ -1321,6 +1351,7 @@ map_variable_attnos(Node *node,
 	context.map_length = map_length;
 	context.to_rowtype = to_rowtype;
 	context.found_whole_row = found_whole_row;
+	context.coerced_var = false;
 
 	*found_whole_row = false;