From 779d48f4de9cf0728ec8a3e99edda8d125a30eff Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <richard@matrix.org>
Date: Sat, 5 Nov 2022 18:21:07 +0000
Subject: [PATCH 1/2] Fix background update table-scanning `events`

When this background update did its last batch, it would try to update all the
events that had been inserted since the bgupdate started, which could cause a
table-scan. Make sure we limit the update correctly
---
 changelog.d/14374.bugfix                         |  1 +
 .../storage/databases/main/events_bg_updates.py  | 16 ++++++++--------
 2 files changed, 9 insertions(+), 8 deletions(-)
 create mode 100644 changelog.d/14374.bugfix

diff --git a/changelog.d/14374.bugfix b/changelog.d/14374.bugfix
new file mode 100644
index 000000000000..8226f5b65118
--- /dev/null
+++ b/changelog.d/14374.bugfix
@@ -0,0 +1 @@
+Fix a background database update which could cause poor database performance.
diff --git a/synapse/storage/databases/main/events_bg_updates.py b/synapse/storage/databases/main/events_bg_updates.py
index 6e8aeed7b4b0..9e31798ab198 100644
--- a/synapse/storage/databases/main/events_bg_updates.py
+++ b/synapse/storage/databases/main/events_bg_updates.py
@@ -1435,16 +1435,16 @@ def _populate_txn(txn: LoggingTransaction) -> bool:
                 ),
             )
 
-            endpoint = None
             row = txn.fetchone()
             if row:
                 endpoint = row[0]
+            else:
+                # if the query didn't return a row, we must be almost done. We just
+                # need to go up to the recorded max_stream_ordering.
+                endpoint = max_stream_ordering_inclusive
 
-            where_clause = "stream_ordering > ?"
-            args = [min_stream_ordering_exclusive]
-            if endpoint:
-                where_clause += " AND stream_ordering <= ?"
-                args.append(endpoint)
+            where_clause = "stream_ordering > ? AND stream_ordering <= ?"
+            args = [min_stream_ordering_exclusive, endpoint]
 
             # now do the updates.
             txn.execute(
@@ -1458,13 +1458,13 @@ def _populate_txn(txn: LoggingTransaction) -> bool:
             )
 
             logger.info(
-                "populated new `events` columns up to %s/%i: updated %i rows",
+                "populated new `events` columns up to %i/%i: updated %i rows",
                 endpoint,
                 max_stream_ordering_inclusive,
                 txn.rowcount,
             )
 
-            if endpoint is None:
+            if endpoint >= max_stream_ordering_inclusive:
                 # we're done
                 return True
 

From 27c456b0159299c95589e611164fdabafee72dad Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Mon, 7 Nov 2022 13:43:43 +0000
Subject: [PATCH 2/2] Update changelog.d/14374.bugfix

---
 changelog.d/14374.bugfix | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/changelog.d/14374.bugfix b/changelog.d/14374.bugfix
index 8226f5b65118..8366cfbf8a2b 100644
--- a/changelog.d/14374.bugfix
+++ b/changelog.d/14374.bugfix
@@ -1 +1 @@
-Fix a background database update which could cause poor database performance.
+Fix a background database update, introduced in Synapse 1.64.0, which could cause poor database performance.