1 files changed, 28 insertions, 0 deletions
diff --git a/synapse/storage/schema/main/delta/68/04_refresh_tokens_index_next_token_id.sql b/synapse/storage/schema/main/delta/68/04_refresh_tokens_index_next_token_id.sql
new file mode 100644
index 0000000000..09305638ea
--- /dev/null
+++ b/synapse/storage/schema/main/delta/68/04_refresh_tokens_index_next_token_id.sql
@@ -0,0 +1,28 @@
+/* Copyright 2022 The Matrix.org Foundation C.I.C
+ *
+ * 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.
+ */
+
+-- next_token_id is a foreign key reference, so previously required a table scan
+-- when a row in the referenced table was deleted.
+-- As it was self-referential and cascaded deletes, this led to O(t*n) time to
+-- delete a row, where t: number of rows in the table and n: number of rows in
+-- the ancestral 'chain' of access tokens.
+--
+-- This index is partial since we only require it for rows which reference
+-- another.
+-- Performance was tested to be the same regardless of whether the index was
+-- full or partial, but a partial index can be smaller.
+CREATE INDEX refresh_tokens_next_token_id
+    ON refresh_tokens(next_token_id)
+    WHERE next_token_id IS NOT NULL;