Add UNIQUE INDEX instead of replacing pkey

This prevents having to break DB compatibility / having to bump
SCHEMA_COMPAT_VERSION.

Use UNIQUE INDEX instead of ALTER TABLE for compatibility with SQLite.

Author: AndrewFerr

synapse/storage/schema/__init__.py

@@ -170,13 +170,13 @@
     - Cleaned up a trigger that was added in #18260 and then reverted.
 
 Changes in SCHEMA_VERSION = 93
-    - MSC4140: Remove `user_localpart` from the primary key of the `delayed_events` table,
-      so that delayed events may be uniquely identified by their `delay_id` alone.
+    - MSC4140: Set delayed events to be uniquely identifiable by their delay ID.
 """
 
 
 SCHEMA_COMPAT_VERSION = (
     # Transitive links are no longer written to `event_auth_chain_links`
+    # TODO: On the next compat bump, update the primary key of `delayed_events`
     84
 )
 """Limit on how far the synapse codebase can be rolled back without breaking db compat

…ta/93/01_add_delayed_events.sql.postgres …/main/delta/93/01_add_delayed_events.sqlsynapse/storage/schema/main/delta/93/01_add_delayed_events.sql.postgres renamed to synapse/storage/schema/main/delta/93/01_add_delayed_events.sql synapse/storage/schema/main/delta/93/01_add_delayed_events.sql.postgres renamed to synapse/storage/schema/main/delta/93/01_add_delayed_events.sql

@@ -11,7 +11,10 @@
 -- See the GNU Affero General Public License for more details:
 -- <https://www.gnu.org/licenses/agpl-3.0.html>.
 
--- Remove `user_localpart` from the primary key of the `delayed_events` table.
+-- Set delayed events to be uniquely identifiable by their delay_id.
 
-ALTER TABLE delayed_events DROP CONSTRAINT delayed_events_pkey;
-ALTER TABLE delayed_events ADD PRIMARY KEY (delay_id);
+-- In practice, delay_ids are already unique because they are generated
+-- from cryptographically strong random strings.
+-- Therefore, adding this constraint is not expected to ever fail,
+-- despite the current pkey technically allowing non-unique delay_ids.
+CREATE UNIQUE INDEX delayed_events_idx ON delayed_events (delay_id);