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: 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 241a7be51e..1a859352b6 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+):")
 
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