perf: add covering index for monitor-bars status aggregation

migrations/20260617120000_add_monitoring_data_covering_index.ts

@@ -0,0 +1,28 @@
+import type { Knex } from "knex";
+
+export async function up(knex: Knex): Promise<void> {
+  // Covering index for the grouped status-count aggregation used by the
+  // monitor-bars dashboard endpoint. The query filters by
+  // (monitor_tag, timestamp range) and reads status + latency for every row.
+  // Including status and latency in the index lets the database satisfy the
+  // query from the index alone, avoiding a heap lookup per matched row.
+  try {
+    await knex.schema.alterTable("monitoring_data", (table) => {
+      table.index(
+        ["monitor_tag", "timestamp", "status", "latency"],
+        "idx_monitoring_data_monitor_tag_timestamp_status_latency",
+      );
+    });
+  } catch (_e) {
+    /* index already exists */
+  }
+}
+
+export async function down(knex: Knex): Promise<void> {
+  await knex.schema.alterTable("monitoring_data", (table) => {
+    table.dropIndex(
+      ["monitor_tag", "timestamp", "status", "latency"],
+      "idx_monitoring_data_monitor_tag_timestamp_status_latency",
+    );
+  });
+}