Allow new values to be added to an existing enum type.

After much expenditure of effort, we've got this to the point where the
performance penalty is pretty minimal in typical cases.

Andrew Dunstan, reviewed by Brendan Jurd, Dean Rasheed, and Tom Lane

src/include/nodes/parsenodes.h

@@ -2193,6 +2193,18 @@ typedef struct CreateEnumStmt
 	List	   *vals;			/* enum values (list of Value strings) */
 } CreateEnumStmt;
 
+/* ----------------------
+ *		Alter Type Statement, enum types
+ * ----------------------
+ */
+typedef struct AlterEnumStmt
+{
+	NodeTag		type;
+	List	   *typeName;		/* qualified name (list of Value strings) */
+	char	   *newVal;			/* new enum value's name */
+	char	   *newValNeighbor;	/* neighboring enum value, if specified */
+	bool	    newValIsAfter;	/* place new enum value after neighbor? */
+} AlterEnumStmt;
 
 /* ----------------------
  *		Create View Statement