Fix thread BG update to not seq scan event_json (#11192)

For some reason the query optimiser decided to seq scan both tables, rather than index scanning `event_json`.
matrix-org · Oct 27, 2021 · 72626b7 · 72626b7
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commit 72626b7
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diff --git a/changelog.d/11192.feature b/changelog.d/11192.feature
@@ -0,0 +1 @@
+Experimental support for the thread relation defined in [MSC3440](https://github.com/matrix-org/matrix-doc/pull/3440).
diff --git a/synapse/storage/databases/main/events_bg_updates.py b/synapse/storage/databases/main/events_bg_updates.py
@@ -1108,7 +1108,7 @@ def _event_thread_relation_txn(txn: LoggingTransaction) -> int:
                 """
                 SELECT event_id, json FROM event_json
                 LEFT JOIN event_relations USING (event_id)
-                WHERE event_id > ? AND relates_to_id IS NULL
+                WHERE event_id > ? AND event_relations.event_id IS NULL
                 ORDER BY event_id LIMIT ?
                 """,
                 (last_event_id, batch_size),
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Experimental support for the thread relation defined in [MSC3440](https://github.com/matrix-org/matrix-doc/pull/3440).