diff --git a/synapse/storage/databases/main/state_deltas.py b/synapse/storage/databases/main/state_deltas.py
new file mode 100644
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+++ b/synapse/storage/databases/main/state_deltas.py
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+# -*- coding: utf-8 -*-
+# Copyright 2018 Vector Creations Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import logging
+
+from twisted.internet import defer
+
+from synapse.storage._base import SQLBaseStore
+
+logger = logging.getLogger(__name__)
+
+
+class StateDeltasStore(SQLBaseStore):
+ def get_current_state_deltas(self, prev_stream_id: int, max_stream_id: int):
+ """Fetch a list of room state changes since the given stream id
+
+ Each entry in the result contains the following fields:
+ - stream_id (int)
+ - room_id (str)
+ - type (str): event type
+ - state_key (str):
+ - event_id (str|None): new event_id for this state key. None if the
+ state has been deleted.
+ - prev_event_id (str|None): previous event_id for this state key. None
+ if it's new state.
+
+ Args:
+ prev_stream_id (int): point to get changes since (exclusive)
+ max_stream_id (int): the point that we know has been correctly persisted
+ - ie, an upper limit to return changes from.
+
+ Returns:
+ Deferred[tuple[int, list[dict]]: A tuple consisting of:
+ - the stream id which these results go up to
+ - list of current_state_delta_stream rows. If it is empty, we are
+ up to date.
+ """
+ prev_stream_id = int(prev_stream_id)
+
+ # check we're not going backwards
+ assert prev_stream_id <= max_stream_id
+
+ if not self._curr_state_delta_stream_cache.has_any_entity_changed(
+ prev_stream_id
+ ):
+ # if the CSDs haven't changed between prev_stream_id and now, we
+ # know for certain that they haven't changed between prev_stream_id and
+ # max_stream_id.
+ return defer.succeed((max_stream_id, []))
+
+ def get_current_state_deltas_txn(txn):
+ # First we calculate the max stream id that will give us less than
+ # N results.
+ # We arbitarily limit to 100 stream_id entries to ensure we don't
+ # select toooo many.
+ sql = """
+ SELECT stream_id, count(*)
+ FROM current_state_delta_stream
+ WHERE stream_id > ? AND stream_id <= ?
+ GROUP BY stream_id
+ ORDER BY stream_id ASC
+ LIMIT 100
+ """
+ txn.execute(sql, (prev_stream_id, max_stream_id))
+
+ total = 0
+
+ for stream_id, count in txn:
+ total += count
+ if total > 100:
+ # We arbitarily limit to 100 entries to ensure we don't
+ # select toooo many.
+ logger.debug(
+ "Clipping current_state_delta_stream rows to stream_id %i",
+ stream_id,
+ )
+ clipped_stream_id = stream_id
+ break
+ else:
+ # if there's no problem, we may as well go right up to the max_stream_id
+ clipped_stream_id = max_stream_id
+
+ # Now actually get the deltas
+ sql = """
+ SELECT stream_id, room_id, type, state_key, event_id, prev_event_id
+ FROM current_state_delta_stream
+ WHERE ? < stream_id AND stream_id <= ?
+ ORDER BY stream_id ASC
+ """
+ txn.execute(sql, (prev_stream_id, clipped_stream_id))
+ return clipped_stream_id, self.db_pool.cursor_to_dict(txn)
+
+ return self.db_pool.runInteraction(
+ "get_current_state_deltas", get_current_state_deltas_txn
+ )
+
+ def _get_max_stream_id_in_current_state_deltas_txn(self, txn):
+ return self.db_pool.simple_select_one_onecol_txn(
+ txn,
+ table="current_state_delta_stream",
+ keyvalues={},
+ retcol="COALESCE(MAX(stream_id), -1)",
+ )
+
+ def get_max_stream_id_in_current_state_deltas(self):
+ return self.db_pool.runInteraction(
+ "get_max_stream_id_in_current_state_deltas",
+ self._get_max_stream_id_in_current_state_deltas_txn,
+ )
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