Allow IF NOT EXISTS when add a new enum label.

If the label is already in the enum the statement becomes a no-op.
This will reduce the pain that comes from our not allowing this
operation inside a transaction block.

Andrew Dunstan, reviewed by Tom Lane and Magnus Hagander.

src/backend/catalog/pg_enum.c

@@ -179,7 +179,8 @@ void
 AddEnumLabel(Oid enumTypeOid,
 			 const char *newVal,
 			 const char *neighbor,
-			 bool newValIsAfter)
+			 bool newValIsAfter,
+	         bool skipIfExists)
 {
 	Relation	pg_enum;
 	Oid			newOid;
@@ -211,6 +212,21 @@ AddEnumLabel(Oid enumTypeOid,
 	 */
 	LockDatabaseObject(TypeRelationId, enumTypeOid, 0, ExclusiveLock);
 
+	/* Do the "IF NOT EXISTS" test if specified */
+	if (skipIfExists)
+	{
+		HeapTuple tup;
+
+		tup = SearchSysCache2(ENUMTYPOIDNAME,
+							  ObjectIdGetDatum(enumTypeOid),
+							  CStringGetDatum(newVal));
+		if (HeapTupleIsValid(tup))
+		{
+			ReleaseSysCache(tup);
+			return;
+		}
+	}
+
 	pg_enum = heap_open(EnumRelationId, RowExclusiveLock);
 
 	/* If we have to renumber the existing members, we restart from here */

src/backend/commands/typecmds.c

@@ -1188,7 +1188,8 @@ AlterEnum(AlterEnumStmt *stmt)
 
 	/* Add the new label */
 	AddEnumLabel(enum_type_oid, stmt->newVal,
-				 stmt->newValNeighbor, stmt->newValIsAfter);
+				 stmt->newValNeighbor, stmt->newValIsAfter, 
+				 stmt->skipIfExists);
 
 	ReleaseSysCache(tup);
 }

src/backend/nodes/copyfuncs.c

@@ -3055,6 +3055,7 @@ _copyAlterEnumStmt(const AlterEnumStmt *from)
 	COPY_STRING_FIELD(newVal);
 	COPY_STRING_FIELD(newValNeighbor);
 	COPY_SCALAR_FIELD(newValIsAfter);
+	COPY_SCALAR_FIELD(skipIfExists);
 
 	return newnode;
 }

src/backend/nodes/equalfuncs.c

@@ -1439,6 +1439,7 @@ _equalAlterEnumStmt(const AlterEnumStmt *a, const AlterEnumStmt *b)
 	COMPARE_STRING_FIELD(newVal);
 	COMPARE_STRING_FIELD(newValNeighbor);
 	COMPARE_SCALAR_FIELD(newValIsAfter);
+	COMPARE_SCALAR_FIELD(skipIfExists);
 
 	return true;
 }

src/backend/parser/gram.y

@@ -470,7 +470,7 @@ static void processCASbits(int cas_bits, int location, const char *constrType,
 %type <windef>	window_definition over_clause window_specification
 				opt_frame_clause frame_extent frame_bound
 %type <str>		opt_existing_window_name
-
+%type <boolean> opt_if_not_exists
 
 /*
  * Non-keyword token types.  These are hard-wired into the "flex" lexer.
@@ -4618,35 +4618,42 @@ enum_val_list:	Sconst
  *****************************************************************************/
 
 AlterEnumStmt:
-		ALTER TYPE_P any_name ADD_P VALUE_P Sconst
+		ALTER TYPE_P any_name ADD_P VALUE_P opt_if_not_exists Sconst
 			{
 				AlterEnumStmt *n = makeNode(AlterEnumStmt);
 				n->typeName = $3;
-				n->newVal = $6;
+				n->newVal = $7;
 				n->newValNeighbor = NULL;
 				n->newValIsAfter = true;
+				n->skipIfExists = $6;
 				$$ = (Node *) n;
 			}
-		 | ALTER TYPE_P any_name ADD_P VALUE_P Sconst BEFORE Sconst
+		 | ALTER TYPE_P any_name ADD_P VALUE_P opt_if_not_exists Sconst BEFORE Sconst
 			{
 				AlterEnumStmt *n = makeNode(AlterEnumStmt);
 				n->typeName = $3;
-				n->newVal = $6;
-				n->newValNeighbor = $8;
+				n->newVal = $7;
+				n->newValNeighbor = $9;
 				n->newValIsAfter = false;
+				n->skipIfExists = $6;
 				$$ = (Node *) n;
 			}
-		 | ALTER TYPE_P any_name ADD_P VALUE_P Sconst AFTER Sconst
+		 | ALTER TYPE_P any_name ADD_P VALUE_P opt_if_not_exists Sconst AFTER Sconst
 			{
 				AlterEnumStmt *n = makeNode(AlterEnumStmt);
 				n->typeName = $3;
-				n->newVal = $6;
-				n->newValNeighbor = $8;
+				n->newVal = $7;
+				n->newValNeighbor = $9;
 				n->newValIsAfter = true;
+				n->skipIfExists = $6;
 				$$ = (Node *) n;
 			}
 		 ;
 
+opt_if_not_exists: IF_P NOT EXISTS              { $$ = true; }
+         | /* empty */                          { $$ = false; }
+         ;
+
 
 /*****************************************************************************
  *

src/include/catalog/pg_enum.h

@@ -65,6 +65,7 @@ typedef FormData_pg_enum *Form_pg_enum;
 extern void EnumValuesCreate(Oid enumTypeOid, List *vals);
 extern void EnumValuesDelete(Oid enumTypeOid);
 extern void AddEnumLabel(Oid enumTypeOid, const char *newVal,
-			 const char *neighbor, bool newValIsAfter);
+						 const char *neighbor, bool newValIsAfter, 
+						 bool skipIfExists);
 
 #endif   /* PG_ENUM_H */

src/include/nodes/parsenodes.h

@@ -2306,6 +2306,7 @@ typedef struct AlterEnumStmt
 	char	   *newVal;			/* new enum value's name */
 	char	   *newValNeighbor; /* neighboring enum value, if specified */
 	bool		newValIsAfter;	/* place new enum value after neighbor? */
+	bool        skipIfExists;   /* ignore statement if label already exists */
 } AlterEnumStmt;
 
 /* ----------------------

src/test/regress/expected/enum.out

@@ -95,6 +95,28 @@ ERROR:  invalid enum label "plutoplutoplutoplutoplutoplutoplutoplutoplutoplutopl
 DETAIL:  Labels must be 63 characters or less.
 ALTER TYPE planets ADD VALUE 'pluto' AFTER 'zeus';
 ERROR:  "zeus" is not an existing enum label
+-- if not exists tests
+--  existing value gives error
+-- We can't do this test because the error contains the
+-- offending Oid value, which is unpredictable.
+-- ALTER TYPE planets ADD VALUE 'mercury';
+-- unless IF NOT EXISTS is specified
+ALTER TYPE planets ADD VALUE IF NOT EXISTS 'mercury';
+-- should be neptune, not mercury
+SELECT enum_last(NULL::planets);
+ enum_last 
+-----------
+ neptune
+(1 row)
+
+ALTER TYPE planets ADD VALUE IF NOT EXISTS 'pluto';
+-- should be pluto, i.e. the new value
+SELECT enum_last(NULL::planets);
+ enum_last 
+-----------
+ pluto
+(1 row)
+
 --
 -- Test inserting so many values that we have to renumber
 --

src/test/regress/sql/enum.sql

@@ -54,6 +54,26 @@ ALTER TYPE planets ADD VALUE
 
 ALTER TYPE planets ADD VALUE 'pluto' AFTER 'zeus';
 
+-- if not exists tests
+
+--  existing value gives error
+
+-- We can't do this test because the error contains the
+-- offending Oid value, which is unpredictable.
+-- ALTER TYPE planets ADD VALUE 'mercury';
+
+-- unless IF NOT EXISTS is specified
+ALTER TYPE planets ADD VALUE IF NOT EXISTS 'mercury';
+
+-- should be neptune, not mercury
+SELECT enum_last(NULL::planets);
+
+ALTER TYPE planets ADD VALUE IF NOT EXISTS 'pluto';
+
+-- should be pluto, i.e. the new value
+SELECT enum_last(NULL::planets);
+
+
 --
 -- Test inserting so many values that we have to renumber
 --