[#514] Do not use postgres concat function. Use || instead.

Concat is not supported in pg 9.0. The || syntax howver does not allow  null as one argument. So we use coalesce to either get the value or an empty string.

ckanext/datastore/plugin.py

@@ -183,7 +183,7 @@ def _check_read_permissions(self):
     def _create_alias_table(self):
         mapping_sql = '''
             SELECT DISTINCT
-                substr(md5(concat(dependee.relname, dependent.relname)), 0, 17) AS "_id",
+                substr(md5(dependee.relname || COALESCE(dependent.relname, '')), 0, 17) AS "_id",
                 dependee.relname AS name,
                 dependee.oid AS oid,
                 dependent.relname AS alias_of