Fix for completed query performance

Author: Jamesbarford

database/src/pool/postgres.rs

@@ -410,6 +410,11 @@ static MIGRATIONS: &[&str] = &[
 
     CREATE INDEX error_artifact_idx ON error(aid);
     "#,
+    // For completed requests we take the last N completed. As the total number
+    // of requests grows to make things fast we need an index on the completed_at
+    r#"
+    CREATE INDEX IF NOT EXISTS benchmark_request_completed_idx ON benchmark_request(completed_at);
+    "#
 ];
 
 #[async_trait::async_trait]