From d69fd53f74f693e0ec756eec479f3d51d93fd2aa Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 2 Oct 2019 11:21:52 +0100
Subject: [PATCH 1/2] Bound find_next_generated_user_id DB query.

We can easily bound the set of user IDs we pull out of the DB, so lets
do that.
---
 synapse/storage/registration.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/synapse/storage/registration.py b/synapse/storage/registration.py
index 241a7be51..1a859352b 100644
--- a/synapse/storage/registration.py
+++ b/synapse/storage/registration.py
@@ -493,7 +493,9 @@ class RegistrationWorkerStore(SQLBaseStore):
         """
 
         def _find_next_generated_user_id(txn):
-            txn.execute("SELECT name FROM users")
+            # We bound between '@1' and '@a' to avoid pulling the entire table
+            # out.
+            txn.execute("SELECT name FROM users WHERE '@1' <= name AND name < '@a'")
 
             regex = re.compile(r"^@(\d+):")
 

From de1823b521cd9b691d060dbdc477be16decdf2af Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 2 Oct 2019 11:23:55 +0100
Subject: [PATCH 2/2] Newsfile

---
 changelog.d/6148.misc | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/6148.misc

diff --git a/changelog.d/6148.misc b/changelog.d/6148.misc
new file mode 100644
index 000000000..1d5213345
--- /dev/null
+++ b/changelog.d/6148.misc
@@ -0,0 +1 @@
+Improve performance of `find_next_generated_user_id` DB query.