Minor corrections for ALTER TYPE ADD VALUE IF NOT EXISTS patch.

Produce a NOTICE when the label already exists, for consistency with other
CREATE IF NOT EXISTS commands.  Also, fix the code so it produces something
more user-friendly than an index violation when the label already exists.
This not incidentally enables making a regression test that the previous
patch didn't make for fear of exposing an unpredictable OID in the results.
Also some wordsmithing on the documentation.

doc/src/sgml/ref/alter_type.sgml

@@ -109,15 +109,16 @@ ALTER TYPE <replaceable class="PARAMETER">name</replaceable> ADD VALUE [ IF NOT
     <term><literal>ADD VALUE [ IF NOT EXISTS ] [ BEFORE | AFTER ]</literal></term>
     <listitem>
      <para>
-      This form adds a new value to an enum type. If the new value's place in
-      the enum's ordering is not specified using <literal>BEFORE</literal> or
-      <literal>AFTER</literal>, then the new item is placed at the end of the
-      list of values.
+      This form adds a new value to an enum type.  The new value's place in
+      the enum's ordering can be specified as being <literal>BEFORE</literal>
+      or <literal>AFTER</literal> one of the existing values.  Otherwise,
+      the new item is added at the end of the list of values.
      </para>
      <para>
-      If <literal>IF NOT EXISTS</literal> is used, it is not an error if the
-      type already contains the new value, and no action  is taken. Otherwise,
-      an error will occur if the new value is already present.
+      If <literal>IF NOT EXISTS</literal> is specified, it is not an error if
+      the type already contains the new value: a notice is issued but no other
+      action is taken. Otherwise, an error will occur if the new value is
+      already present.
      </para>
     </listitem>
    </varlistentry>

src/backend/catalog/pg_enum.c

@@ -212,19 +212,30 @@ AddEnumLabel(Oid enumTypeOid,
 	 */
 	LockDatabaseObject(TypeRelationId, enumTypeOid, 0, ExclusiveLock);
 
-	/* Do the "IF NOT EXISTS" test if specified */
-	if (skipIfExists)
+	/*
+	 * Check if label is already in use.  The unique index on pg_enum would
+	 * catch this anyway, but we prefer a friendlier error message, and
+	 * besides we need a check to support IF NOT EXISTS.
+	 */
+	enum_tup = SearchSysCache2(ENUMTYPOIDNAME,
+							   ObjectIdGetDatum(enumTypeOid),
+							   CStringGetDatum(newVal));
+	if (HeapTupleIsValid(enum_tup))
 	{
-		HeapTuple tup;
-
-		tup = SearchSysCache2(ENUMTYPOIDNAME,
-							  ObjectIdGetDatum(enumTypeOid),
-							  CStringGetDatum(newVal));
-		if (HeapTupleIsValid(tup))
+		ReleaseSysCache(enum_tup);
+		if (skipIfExists)
 		{
-			ReleaseSysCache(tup);
+			ereport(NOTICE,
+					(errcode(ERRCODE_DUPLICATE_OBJECT),
+					 errmsg("enum label \"%s\" already exists, skipping",
+							newVal)));
 			return;
 		}
+		else
+			ereport(ERROR,
+					(errcode(ERRCODE_DUPLICATE_OBJECT),
+					 errmsg("enum label \"%s\" already exists",
+							newVal)));
 	}
 
 	pg_enum = heap_open(EnumRelationId, RowExclusiveLock);

src/include/nodes/parsenodes.h

@@ -2306,7 +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 */
+	bool		skipIfExists;	/* no error if label already exists */
 } AlterEnumStmt;
 
 /* ----------------------

src/test/regress/expected/enum.out

@@ -97,11 +97,11 @@ 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';
+ALTER TYPE planets ADD VALUE 'mercury';
+ERROR:  enum label "mercury" already exists
 -- unless IF NOT EXISTS is specified
 ALTER TYPE planets ADD VALUE IF NOT EXISTS 'mercury';
+NOTICE:  enum label "mercury" already exists, skipping
 -- should be neptune, not mercury
 SELECT enum_last(NULL::planets);
  enum_last 

src/test/regress/sql/enum.sql

@@ -57,10 +57,7 @@ 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';
+ALTER TYPE planets ADD VALUE 'mercury';
 
 -- unless IF NOT EXISTS is specified
 ALTER TYPE planets ADD VALUE IF NOT EXISTS 'mercury';
@@ -73,7 +70,6 @@ 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
 --