diff --git a/packages/db/src/schema/planner.ts b/packages/db/src/schema/planner.ts
index d789283..a797ef4 100644
--- a/packages/db/src/schema/planner.ts
+++ b/packages/db/src/schema/planner.ts
@@ -1,4 +1,4 @@
-import { pgSchema, text, timestamp, boolean, customType } from "drizzle-orm/pg-core";
+import { pgSchema, text, timestamp, boolean, customType, index } from "drizzle-orm/pg-core";
 
 const bytea = customType<{ data: Buffer }>({
   dataType() {
@@ -16,4 +16,9 @@ export const sessions = plannerSchema.table("sessions", {
   createdAt: timestamp("created_at", { withTimezone: true }).notNull().defaultNow(),
   lastActivity: timestamp("last_activity", { withTimezone: true }).notNull().defaultNow(),
   closed: boolean("closed").notNull().default(false),
-});
+}, (t) => ({
+  // Hourly `expireSessions()` job runs `DELETE WHERE last_activity < cutoff`.
+  // Without this index it's a full table scan that grows linearly with
+  // total sessions ever created.
+  lastActivityIdx: index("sessions_last_activity_idx").on(t.lastActivity),
+}));