Identity columns

This is the SQL standard-conforming variant of PostgreSQL's serial
columns.  It fixes a few usability issues that serial columns have:

- CREATE TABLE / LIKE copies default but refers to same sequence
- cannot add/drop serialness with ALTER TABLE
- dropping default does not drop sequence
- need to grant separate privileges to sequence
- other slight weirdnesses because serial is some kind of special macro

Reviewed-by: Vitaly Burovoy <vitaly.burovoy@gmail.com>

src/backend/utils/cache/lsyscache.c

@@ -836,6 +836,38 @@ get_attnum(Oid relid, const char *attname)
 		return InvalidAttrNumber;
 }
 
+/*
+ * get_attidentity
+ *
+ *		Given the relation id and the attribute name,
+ *		return the "attidentity" field from the attribute relation.
+ *
+ *		Returns '\0' if not found.
+ *
+ *		Since no identity is represented by '\0', this can also be used as a
+ *		Boolean test.
+ */
+char
+get_attidentity(Oid relid, AttrNumber attnum)
+{
+	HeapTuple	tp;
+
+	tp = SearchSysCache2(ATTNUM,
+						 ObjectIdGetDatum(relid),
+						 Int16GetDatum(attnum));
+	if (HeapTupleIsValid(tp))
+	{
+		Form_pg_attribute att_tup = (Form_pg_attribute) GETSTRUCT(tp);
+		char			result;
+
+		result = att_tup->attidentity;
+		ReleaseSysCache(tp);
+		return result;
+	}
+	else
+		return '\0';
+}
+
 /*
  * get_atttype
  *

src/backend/utils/cache/relcache.c

@@ -3268,6 +3268,7 @@ RelationBuildLocalRelation(const char *relname,
 	has_not_null = false;
 	for (i = 0; i < natts; i++)
 	{
+		rel->rd_att->attrs[i]->attidentity = tupDesc->attrs[i]->attidentity;
 		rel->rd_att->attrs[i]->attnotnull = tupDesc->attrs[i]->attnotnull;
 		has_not_null |= tupDesc->attrs[i]->attnotnull;
 	}