From 86e7a6d16ee9ffe8f5e783ec8150405b13f878fa Mon Sep 17 00:00:00 2001
From: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>
Date: Fri, 10 Dec 2021 14:13:23 +0000
Subject: [PATCH] Stop populating `state_events.prev_state` (#11558)

this field is never read, so we may as well stop populating it.
---
 changelog.d/11558.misc                   | 1 +
 synapse/storage/databases/main/events.py | 4 ----
 synapse/storage/schema/__init__.py       | 5 ++++-
 3 files changed, 5 insertions(+), 5 deletions(-)
 create mode 100644 changelog.d/11558.misc

diff --git a/changelog.d/11558.misc b/changelog.d/11558.misc
new file mode 100644
index 000000000..7c334f17e
--- /dev/null
+++ b/changelog.d/11558.misc
@@ -0,0 +1 @@
+Stop populating unused database column `state_events.prev_state`.
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index f1f4ce5e0..eed453d83 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -1410,10 +1410,6 @@ class PersistEventsStore:
                 "state_key": event.state_key,
             }
 
-            # TODO: How does this work with backfilling?
-            if hasattr(event, "replaces_state"):
-                vals["prev_state"] = event.replaces_state
-
             state_values.append(vals)
 
         self.db_pool.simple_insert_many_txn(
diff --git a/synapse/storage/schema/__init__.py b/synapse/storage/schema/__init__.py
index 50d08094d..2a3d47185 100644
--- a/synapse/storage/schema/__init__.py
+++ b/synapse/storage/schema/__init__.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-SCHEMA_VERSION = 66  # remember to update the list below when updating
+SCHEMA_VERSION = 67  # remember to update the list below when updating
 """Represents the expectations made by the codebase about the database schema
 
 This should be incremented whenever the codebase changes its requirements on the
@@ -50,6 +50,9 @@ Changes in SCHEMA_VERSION = 65:
 Changes in SCHEMA_VERSION = 66:
     - Queries on state_key columns are now disambiguated (ie, the codebase can handle
       the `events` table having a `state_key` column).
+
+Changes in SCHEMA_VERSION = 67:
+    - state_events.prev_state is no longer written to.
 """