diff --git a/synapse/storage/data_stores/main/stream.py b/synapse/storage/data_stores/main/stream.py
index bb70a0f38a..064f602a65 100644
--- a/synapse/storage/data_stores/main/stream.py
+++ b/synapse/storage/data_stores/main/stream.py
@@ -871,23 +871,38 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
args.append(int(limit))
- # Using DISTINCT in this SELECT query is quite expensive, because it requires the
- # engine to sort on the entire (not limited) result set, i.e. the entire events
- # table. We only need to use it when we're filtering on more than two labels,
- # because that's the only scenario in which we can possibly to get multiple times
- # the same event ID in the results.
select_keywords = "SELECT"
- if event_filter and event_filter.labels and len(event_filter.labels) > 1:
- select_keywords += "DISTINCT"
+ join_clause = ""
+ if event_filter and event_filter.labels:
+ # If we're not filtering on a label, then joining on event_labels will
+ # return as many row for a single event as the number of labels it has. To
+ # avoid this, only join if we're filtering on at least one label.
+ join_clause = (
+ "LEFT JOIN event_labels"
+ " USING (event_id, room_id, topological_ordering)"
+ )
+ if len(event_filter.labels) > 1:
+ # Using DISTINCT in this SELECT query is quite expensive, because it
+ # requires the engine to sort on the entire (not limited) result set,
+ # i.e. the entire events table. We only need to use it when we're
+ # filtering on more than two labels, because that's the only scenario
+ # in which we can possibly to get multiple times the same event ID in
+ # the results.
+ select_keywords += "DISTINCT"
sql = (
"%(select_keywords)s event_id, topological_ordering, stream_ordering"
" FROM events"
- " LEFT JOIN event_labels USING (event_id, room_id, topological_ordering)"
+ " %(join_clause)s"
" WHERE outlier = ? AND room_id = ? AND %(bounds)s"
" ORDER BY topological_ordering %(order)s,"
" stream_ordering %(order)s LIMIT ?"
- ) % {"select_keywords": select_keywords, "bounds": bounds, "order": order}
+ ) % {
+ "select_keywords": select_keywords,
+ "join_clause": join_clause,
+ "bounds": bounds,
+ "order": order
+ }
txn.execute(sql, args)
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