Only join on event_labels if we're filtering on labels

1 files changed, 24 insertions, 9 deletions

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)