[barodeur]

Checking whether a username is available looks users up case-insensitively, with WHERE lower(name) = lower(?). But there is currently no index on lower(name), so each check scans the whole table.

This backs registration, GET /_synapse/admin/v1/username_available and GET /_synapse/mas/is_localpart_available, which Matrix Authentication Service will call as the user types in its registration form (Interactive feedback for username availability in the registration form (element-hq/matrix-authentication-service#5977)).

• Adds users_lower_name on users (lower(name)), built as a background update, on both PostgreSQL and SQLite.
• matrix.org already has an index with this name and definition, created by hand (Postgres indexes differ on matrix.org (matrix-org/synapse#11893)). The background update drops and rebuilds it concurrently rather than adding a duplicate; lookups fall back to a table scan until the rebuild finishes.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct (run the linters)

[github-actions]

Schema Diff

Please check that this looks as expected!

common (sqlite)

Unchanged

common (postgres)

Unchanged

main (sqlite)

--- /tmp/schema_diff_dkflz9tq/before/main/full_schemas/9999/full.sql.sqlite	2026-10-16 13:18:22.144218654 +0000
+++ /tmp/schema_diff_dkflz9tq/after/main/full_schemas/9999/full.sql.sqlite	2026-10-16 13:18:09.690286674 +0000
@@ -1253,20 +1253,21 @@
 CREATE INDEX sliding_sync_membership_snapshots_membership_event_id_idx ON sliding_sync_membership_snapshots (membership_event_id) ;
 CREATE INDEX sliding_sync_membership_snapshots_user_id_stream_ordering ON sliding_sync_membership_snapshots (user_id, event_stream_ordering) ;
 CREATE INDEX device_federation_inbox_received_ts_index ON device_federation_inbox (received_ts) ;
 CREATE UNIQUE INDEX event_txn_id_device_id_txn_id2 ON event_txn_id_device_id (user_id, device_id, room_id, txn_id) ;
 CREATE UNIQUE INDEX delayed_events_idx ON delayed_events (delay_id) ;
 CREATE INDEX device_lists_changes_in_room_inserted_ts_idx ON device_lists_changes_in_room (inserted_ts) WHERE inserted_ts IS NOT NULL;
 CREATE INDEX sliding_sync_connections_last_used_ts_idx ON sliding_sync_connections (last_used_ts) WHERE last_used_ts IS NOT NULL;
 CREATE INDEX sliding_sync_connection_lazy_members_conn_pos_idx ON sliding_sync_connection_lazy_members (connection_position) WHERE connection_position IS NOT NULL;
 CREATE INDEX device_lists_changes_in_room_unconverted_idx ON device_lists_changes_in_room (user_id, device_id, room_id, stream_id) WHERE NOT converted_to_destinations;
 CREATE UNIQUE INDEX e2e_cross_signing_signatures_idx3 ON e2e_cross_signing_signatures (user_id, target_user_id, target_device_id, key_id) ;
+CREATE INDEX users_lower_name ON users (lower(name)) ;
 /* WARNING: Script requires that SQLITE_DBCONFIG_DEFENSIVE be disabled */
 INSERT INTO appservice_stream_position VALUES('X',0);
 INSERT INTO federation_stream_position VALUES('federation',-1,'master');
 INSERT INTO federation_stream_position VALUES('events',-1,'master');
 INSERT INTO event_push_summary_stream_ordering VALUES('X',0);
 INSERT INTO user_directory_stream_pos VALUES('X',-1);
 INSERT INTO stats_incremental_position VALUES('X',1);
 INSERT INTO event_push_summary_last_receipt_stream_id VALUES('X',0);
 INSERT INTO device_lists_changes_converted_stream_position VALUES('X',1,'',NULL);
 INSERT INTO room_forgetter_stream_pos VALUES('X',0);

main (postgres)

--- /tmp/schema_diff_dkflz9tq/before/main/full_schemas/9999/full.sql.postgres	2026-10-16 13:18:22.703215783 +0000
+++ /tmp/schema_diff_dkflz9tq/after/main/full_schemas/9999/full.sql.postgres	2026-10-16 13:18:10.360283287 +0000
@@ -1838,20 +1838,21 @@
 CREATE INDEX user_reports_target_user_id ON user_reports USING btree (target_user_id);
 CREATE INDEX user_reports_user_id ON user_reports USING btree (user_id);
 CREATE UNIQUE INDEX user_signature_stream_idx ON user_signature_stream USING btree (stream_id);
 CREATE UNIQUE INDEX user_threepid_id_server_idx ON user_threepid_id_server USING btree (user_id, medium, address, id_server);
 CREATE INDEX user_threepids_medium_address ON user_threepids USING btree (medium, address);
 CREATE INDEX user_threepids_user_id ON user_threepids USING btree (user_id);
 CREATE INDEX users_creation_ts ON users USING btree (creation_ts);
 CREATE INDEX users_have_local_media ON local_media_repository USING btree (user_id, created_ts);
 CREATE INDEX users_in_public_rooms_r_idx ON users_in_public_rooms USING btree (room_id);
 CREATE UNIQUE INDEX users_in_public_rooms_u_idx ON users_in_public_rooms USING btree (user_id, room_id);
+CREATE INDEX users_lower_name ON users USING btree (lower(name));
 CREATE INDEX users_who_share_private_rooms_o_idx ON users_who_share_private_rooms USING btree (other_user_id);
 CREATE INDEX users_who_share_private_rooms_r_idx ON users_who_share_private_rooms USING btree (room_id);
 CREATE UNIQUE INDEX users_who_share_private_rooms_u_idx ON users_who_share_private_rooms USING btree (user_id, other_user_id, room_id);
 CREATE UNIQUE INDEX worker_locks_key ON worker_locks USING btree (lock_name, lock_key);
 CREATE UNIQUE INDEX worker_read_write_locks_key ON worker_read_write_locks USING btree (lock_name, lock_key, token);
 CREATE UNIQUE INDEX worker_read_write_locks_mode_key ON worker_read_write_locks_mode USING btree (lock_name, lock_key);
 CREATE UNIQUE INDEX worker_read_write_locks_mode_type ON worker_read_write_locks_mode USING btree (lock_name, lock_key, write_lock);
 CREATE UNIQUE INDEX worker_read_write_locks_write ON worker_read_write_locks USING btree (lock_name, lock_key) WHERE write_lock;
 CREATE TRIGGER check_event_stream_ordering BEFORE INSERT OR UPDATE ON current_state_events FOR EACH ROW EXECUTE FUNCTION check_event_stream_ordering();
 CREATE TRIGGER check_event_stream_ordering BEFORE INSERT OR UPDATE ON local_current_membership FOR EACH ROW EXECUTE FUNCTION check_event_stream_ordering();

state (sqlite)

Unchanged

state (postgres)

Unchanged

[sandhose]

That feels a bit excessive to parse the query plan to make sure the index is being used? Do we really need to test any of this?

[barodeur]

removed in a0bbba9

[sandhose]

I was a little concerned that on m.org we would first drop the existing index then spend 20min recreating it, and have to do seq scan during that time. To avoid this, I renamed the index on Matrix.org, and we'll need to remember to remove it later:

ALTER INDEX users_lower_name RENAME TO temp_quenting_pr20369_users_lower_name;