Replace postgres GIN with GIST

This is because GIN can be slow to write too, especially when the table
gets large.

synapse/storage/prepare_database.py

@@ -25,7 +25,7 @@ logger = logging.getLogger(__name__)
 
 # Remember to update this number every time a change is made to database
 # schema files, so the users will be informed on server restarts.
-SCHEMA_VERSION = 37
+SCHEMA_VERSION = 38
 
 dir_path = os.path.abspath(os.path.dirname(__file__))
 

synapse/storage/schema/delta/38/postgres_fts_gist.sql

@@ -0,0 +1,17 @@
+/* Copyright 2016 OpenMarket Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+ INSERT into background_updates (update_name, progress_json)
+     VALUES ('event_search_postgres_gist', '{}');

synapse/storage/search.py

@@ -31,6 +31,7 @@ class SearchStore(BackgroundUpdateStore):
 
     EVENT_SEARCH_UPDATE_NAME = "event_search"
     EVENT_SEARCH_ORDER_UPDATE_NAME = "event_search_order"
+    EVENT_SEARCH_USE_GIST_POSTGRES_NAME = "event_search_postgres_gist"
 
     def __init__(self, hs):
         super(SearchStore, self).__init__(hs)
@@ -41,6 +42,10 @@ class SearchStore(BackgroundUpdateStore):
             self.EVENT_SEARCH_ORDER_UPDATE_NAME,
             self._background_reindex_search_order
         )
+        self.register_background_update_handler(
+            self.EVENT_SEARCH_USE_GIST_POSTGRES_NAME,
+            self._background_reindex_gist_search
+        )
 
     @defer.inlineCallbacks
     def _background_reindex_search(self, progress, batch_size):
@@ -139,6 +144,28 @@ class SearchStore(BackgroundUpdateStore):
 
         defer.returnValue(result)
 
+    @defer.inlineCallbacks
+    def _background_reindex_gist_search(self, progress, batch_size):
+        def create_index(conn):
+            conn.rollback()
+            conn.set_session(autocommit=True)
+            c = conn.cursor()
+
+            # We create with NULLS FIRST so that when we search *backwards*
+            # we get the ones with non null origin_server_ts *first*
+            c.execute(
+                "CREATE INDEX CONCURRENTLY event_search_fts_idx_gist"
+                " ON event_search USING GIST (vector)"
+            )
+            c.execute("DROP INDEX event_search_fts_idx")
+            conn.set_session(autocommit=False)
+
+        if isinstance(self.database_engine, PostgresEngine):
+            yield self.runWithConnection(create_index)
+
+        yield self._end_background_update(self.EVENT_SEARCH_USE_GIST_POSTGRES_NAME)
+        defer.returnValue(1)
+
     @defer.inlineCallbacks
     def _background_reindex_search_order(self, progress, batch_size):
         target_min_stream_id = progress["target_min_stream_id_inclusive"]