Fix perf of get_cross_signing_keys (#9116)

changelog.d/9114.bugfix

@@ -1 +1 @@
-Fix bug in federation catchup logic that caused outbound federation to be delayed for large servers after start up. Introduced in v1.21.0.
+Fix bugs in federation catchup logic that caused outbound federation to be delayed for large servers after start up. Introduced in v1.8.0 and v1.21.0.

changelog.d/9116.bugfix

@@ -0,0 +1 @@
+Fix bugs in federation catchup logic that caused outbound federation to be delayed for large servers after start up. Introduced in v1.8.0 and v1.21.0.

synapse/storage/databases/main/end_to_end_keys.py

@@ -25,6 +25,7 @@
 from synapse.logging.opentracing import log_kv, set_tag, trace
 from synapse.storage._base import SQLBaseStore, db_to_json
 from synapse.storage.database import DatabasePool, make_in_list_sql_clause
+from synapse.storage.engines import PostgresEngine
 from synapse.storage.types import Cursor
 from synapse.types import JsonDict
 from synapse.util import json_encoder
@@ -513,21 +514,35 @@ def _get_bare_e2e_cross_signing_keys_bulk_txn(
 
         for user_chunk in batch_iter(user_ids, 100):
             clause, params = make_in_list_sql_clause(
-                txn.database_engine, "k.user_id", user_chunk
-            )
-            sql = (
-                """
-                SELECT k.user_id, k.keytype, k.keydata, k.stream_id
-                  FROM e2e_cross_signing_keys k
-                  INNER JOIN (SELECT user_id, keytype, MAX(stream_id) AS stream_id
-                                FROM e2e_cross_signing_keys
-                               GROUP BY user_id, keytype) s
-                 USING (user_id, stream_id, keytype)
-                 WHERE
-            """
-                + clause
+                txn.database_engine, "user_id", user_chunk
             )
 
+            # Fetch the latest key for each type per user.
+            if isinstance(self.database_engine, PostgresEngine):
+                # The `DISTINCT ON` clause will pick the *first* row it
+                # encounters, so ordering by stream ID desc will ensure we get
+                # the latest key.
+                sql = """
+                    SELECT DISTINCT ON (user_id, keytype) user_id, keytype, keydata, stream_id
+                        FROM e2e_cross_signing_keys
+                        WHERE %(clause)s
+                        ORDER BY user_id, keytype, stream_id DESC
+                """ % {
+                    "clause": clause
+                }
+            else:
+                # SQLite has special handling for bare columns when using
+                # MIN/MAX with a `GROUP BY` clause where it picks the value from
+                # a row that matches the MIN/MAX.
+                sql = """
+                    SELECT user_id, keytype, keydata, MAX(stream_id)
+                        FROM e2e_cross_signing_keys
+                        WHERE %(clause)s
+                        GROUP BY user_id, keytype
+                """ % {
+                    "clause": clause
+                }
+
             txn.execute(sql, params)
             rows = self.db_pool.cursor_to_dict(txn)