Fixed #18330 - Made cache culling 3rd party db backend friendly

This is Ian Kelly's patch from #15580 with minor modifications.

django/core/cache/backends/db.py

@@ -167,17 +167,9 @@ class DatabaseCache(BaseDatabaseCache):
             num = cursor.fetchone()[0]
             if num > self._max_entries:
                 cull_num = num / self._cull_frequency
-                if connections[db].vendor == 'oracle':
-                    # Oracle doesn't support LIMIT + OFFSET
-                    cursor.execute("""SELECT cache_key FROM
-(SELECT ROW_NUMBER() OVER (ORDER BY cache_key) AS counter, cache_key FROM %s)
-WHERE counter > %%s AND COUNTER <= %%s""" % table, [cull_num, cull_num + 1])
-                else:
-                    # This isn't standard SQL, it's likely to break
-                    # with some non officially supported databases
-                    cursor.execute("SELECT cache_key FROM %s "
-                                   "ORDER BY cache_key "
-                                   "LIMIT 1 OFFSET %%s" % table, [cull_num])
+                cursor.execute(
+                    connections[db].ops.cache_key_culling_sql() % table,
+                    [cull_num])
                 cursor.execute("DELETE FROM %s "
                                "WHERE cache_key < %%s" % table,
                                [cursor.fetchone()[0]])

django/db/backends/__init__.py

@@ -475,6 +475,16 @@ class BaseDatabaseOperations(object):
         """
         return None
 
+    def cache_key_culling_sql(self):
+        """
+        Returns a SQL query that retrieves the first cache key greater than the
+        n smallest.
+
+        This is used by the 'db' cache backend to determine where to start
+        culling.
+        """
+        return "SELECT cache_key FROM %s ORDER BY cache_key LIMIT 1 OFFSET %%s"
+
     def date_extract_sql(self, lookup_type, field_name):
         """
         Given a lookup_type of 'year', 'month' or 'day', returns the SQL that

django/db/backends/oracle/base.py

@@ -118,6 +118,13 @@ WHEN (new.%(col_name)s IS NULL)
 /""" % locals()
         return sequence_sql, trigger_sql
 
+    def cache_key_culling_sql(self):
+        return """
+            SELECT cache_key
+              FROM (SELECT cache_key, rank() OVER (ORDER BY cache_key) AS rank FROM %s)
+             WHERE rank = %%s + 1
+        """
+
     def date_extract_sql(self, lookup_type, field_name):
         # http://download-east.oracle.com/docs/cd/B10501_01/server.920/a96540/functions42a.htm#1017163
         if lookup_type == 'week_day':