diff --git a/ddl/migrations/0218_api_keys_lower_idx.sql b/ddl/migrations/0218_api_keys_lower_idx.sql
new file mode 100644
index 00000000..7b613e00
--- /dev/null
+++ b/ddl/migrations/0218_api_keys_lower_idx.sql
@@ -0,0 +1,5 @@
+-- Rate-limit lookups use LOWER(api_key) = LOWER($1). Production has mixed-case
+-- API keys, so preserve case-insensitive semantics while avoiding repeated
+-- seq scans of api_keys.
+CREATE INDEX CONCURRENTLY IF NOT EXISTS api_keys_lower_api_key_idx
+    ON api_keys (LOWER(api_key));