diff --git a/changelog.d/8396.feature b/changelog.d/8396.feature
new file mode 100644
index 0000000000000000000000000000000000000000..b363e929ea8cb0232932f5ab5f26f22744e81583
--- /dev/null
+++ b/changelog.d/8396.feature
@@ -0,0 +1 @@
+Add experimental support for sharding event persister.
diff --git a/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres b/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres
index 97c1e6a0c5d7a49cca40502452a1901932f7db04..c31f9af82a0f66cb9484155fa8bfa9ba000c24b8 100644
--- a/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres
+++ b/synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres
@@ -21,6 +21,8 @@ SELECT setval('events_stream_seq', (
 
 CREATE SEQUENCE IF NOT EXISTS events_backfill_stream_seq;
 
+-- If the server has never backfilled a room then doing `-MIN(...)` will give
+-- a negative result, hence why we do `GREATEST(...)`
 SELECT setval('events_backfill_stream_seq', (
-    SELECT COALESCE(-MIN(stream_ordering), 1) FROM events
+    SELECT GREATEST(COALESCE(-MIN(stream_ordering), 1), 1) FROM events
 ));
diff --git a/synapse/storage/util/id_generators.py b/synapse/storage/util/id_generators.py
index 727fcc521c8b4e3e140dfb4cf2705ed8df4221cf..4269eaf9187e1921c2f39f01a68e532b5f8e0e10 100644
--- a/synapse/storage/util/id_generators.py
+++ b/synapse/storage/util/id_generators.py
@@ -287,8 +287,12 @@ class MultiWriterIdGenerator:
         min_stream_id = min(self._current_positions.values(), default=None)
 
         if min_stream_id is None:
+            # We add a GREATEST here to ensure that the result is always
+            # positive. (This can be a problem for e.g. backfill streams where
+            # the server has never backfilled).
             sql = """
-                SELECT COALESCE(%(agg)s(%(id)s), 1) FROM %(table)s
+                SELECT GREATEST(COALESCE(%(agg)s(%(id)s), 1), 1)
+                FROM %(table)s
             """ % {
                 "id": id_column,
                 "table": table,