winery: add index for per-shard statistics

swh/objstorage/backends/winery/sharedbase.py

@@ -134,6 +134,10 @@ class SharedBase(Database):
         CREATE INDEX IF NOT EXISTS signature2shard_deleted
             ON signature2shard(signature, shard)
          WHERE state = 'deleted'
+        """,
+            """\
+        CREATE INDEX IF NOT EXISTS signature2shard_shard_state
+            ON signature2shard(shard, state)
         """,
         ]