Add replaces_index parameter to register_background_index_update

synapse/storage/background_updates.py

@@ -518,6 +518,7 @@ def register_background_index_update(
         where_clause: Optional[str] = None,
         unique: bool = False,
         psql_only: bool = False,
+        replaces_index: Optional[str] = None,
     ) -> None:
         """Helper for store classes to do a background index addition
 
@@ -537,6 +538,8 @@ def register_background_index_update(
             unique: true to make a UNIQUE index
             psql_only: true to only create this index on psql databases (useful
                 for virtual sqlite tables)
+            replaces_index: The name of an index that this index replaces.
+                The named index will be dropped upon completion of the new index.
         """
 
         def create_index_psql(conn: Connection) -> None:
@@ -568,6 +571,12 @@ def create_index_psql(conn: Connection) -> None:
                 }
                 logger.debug("[SQL] %s", sql)
                 c.execute(sql)
+
+                if replaces_index is not None:
+                    # We drop the old index as the new index has now been created.
+                    sql = f"DROP INDEX IF EXISTS {replaces_index}"
+                    logger.debug("[SQL] %s", sql)
+                    c.execute(sql)
             finally:
                 conn.set_session(autocommit=False)  # type: ignore
 
@@ -596,6 +605,12 @@ def create_index_sqlite(conn: Connection) -> None:
             logger.debug("[SQL] %s", sql)
             c.execute(sql)
 
+            if replaces_index is not None:
+                # We drop the old index as the new index has now been created.
+                sql = f"DROP INDEX IF EXISTS {replaces_index}"
+                logger.debug("[SQL] %s", sql)
+                c.execute(sql)
+
         if isinstance(self.db_pool.engine, engines.PostgresEngine):
             runner: Optional[Callable[[Connection], None]] = create_index_psql
         elif psql_only: