diff --git a/src/jogasaki/executor/process/impl/ops/scan.cpp b/src/jogasaki/executor/process/impl/ops/scan.cpp
index 82180d85..ed2ec476 100644
--- a/src/jogasaki/executor/process/impl/ops/scan.cpp
+++ b/src/jogasaki/executor/process/impl/ops/scan.cpp
@@ -211,9 +211,11 @@ operation_status scan::operator()(  //NOLINT(readability-function-cognitive-comp
             loop_count = 0;
             auto current_time = std::chrono::steady_clock::now();
             auto elapsed_time =
-            std::chrono::duration_cast<std::chrono::milliseconds>(current_time - previous_time);
-            if (elapsed_time.count() >= scan_yield_interval ) {
-               return {operation_status_kind::yield};
+                std::chrono::duration_cast<std::chrono::milliseconds>(current_time - previous_time);
+            if (elapsed_time.count() >= scan_yield_interval) {
+                ++ctx.yield_count_;
+                VLOG_LP(log_trace) << "scan operator yields count:" << ctx.yield_count_;
+                return {operation_status_kind::yield};
             }
         }
         loop_count++;
diff --git a/src/jogasaki/executor/process/impl/ops/scan_context.h b/src/jogasaki/executor/process/impl/ops/scan_context.h
index 330866c7..712f40d2 100644
--- a/src/jogasaki/executor/process/impl/ops/scan_context.h
+++ b/src/jogasaki/executor/process/impl/ops/scan_context.h
@@ -77,6 +77,7 @@ class scan_context : public context_base {
     impl::scan_info const* scan_info_{};
     data::aligned_buffer key_begin_{};
     data::aligned_buffer key_end_{};
+    std::size_t yield_count_{};
 };
 
 }
diff --git a/src/jogasaki/scheduler/flat_task.cpp b/src/jogasaki/scheduler/flat_task.cpp
index e1ea3524..071c91e3 100644
--- a/src/jogasaki/scheduler/flat_task.cpp
+++ b/src/jogasaki/scheduler/flat_task.cpp
@@ -145,6 +145,8 @@ bool flat_task::execute(tateyama::task_scheduler::context& ctx) {
     if (VLOG_IS_ON(log_debug_timing_event_fine)) {
         begin = clock::now();
     }
+    VLOG_LP(log_trace) << "task begin " << *this << " job_id:" << utils::hex(req_context_->job()->id())
+                       << " kind:" << kind_ << " sticky:" << sticky_;
     bool ret = false;
     switch(kind_) {
         using kind = flat_task_kind;
@@ -157,11 +159,13 @@ bool flat_task::execute(tateyama::task_scheduler::context& ctx) {
         case kind::load: load(); break;
     }
     std::chrono::time_point<clock> end{};
+    std::size_t took_ns{};
     if (VLOG_IS_ON(log_debug_timing_event_fine)) {
         end = clock::now();
+        took_ns = std::chrono::duration_cast<std::chrono::nanoseconds>(end-begin).count();
     }
     if(auto req_detail = job()->request()) {
-        req_detail->task_duration_ns() += std::chrono::duration_cast<std::chrono::nanoseconds>(end-begin).count();
+        req_detail->task_duration_ns() += took_ns;
         ++req_detail->task_count();
         if(sticky_) {
             ++req_detail->sticky_task_count();
@@ -170,6 +174,8 @@ bool flat_task::execute(tateyama::task_scheduler::context& ctx) {
             ++req_detail->task_steling_count();
         }
     }
+    VLOG_LP(log_trace) << "task end " << *this << " took(ns):" << took_ns
+                       << " job_id:" << utils::hex(req_context_->job()->id()) << " kind:" << kind_;
     return ret;
 }
 
@@ -260,7 +266,7 @@ flat_task::identity_type flat_task::id() const {
     if (origin_) {
         return origin_->id();
     }
-    return undefined_id;
+    return id_;
 }
 
 flat_task::flat_task(
@@ -278,6 +284,7 @@ flat_task::flat_task(
     task_enum_tag_t<flat_task_kind::dag_events>,
     request_context* rctx
 ) noexcept:
+    id_(id_src_++),
     kind_(flat_task_kind::dag_events),
     req_context_(rctx)
 {}
@@ -287,6 +294,7 @@ flat_task::flat_task(
     request_context* rctx,
     model::graph& g
 ) noexcept:
+    id_(id_src_++),
     kind_(flat_task_kind::bootstrap),
     req_context_(rctx),
     graph_(std::addressof(g))
@@ -335,6 +343,7 @@ flat_task::flat_task(
     task_enum_tag_t<flat_task_kind::teardown>,
     request_context* rctx
 ) noexcept:
+    id_(id_src_++),
     kind_(flat_task_kind::teardown),
     req_context_(rctx)
 {}
@@ -344,6 +353,7 @@ flat_task::flat_task(
     request_context* rctx,
     executor::common::write_statement* write
 ) noexcept:
+    id_(id_src_++),
     kind_(flat_task_kind::write),
     req_context_(rctx),
     write_(write),
@@ -363,6 +373,7 @@ flat_task::flat_task(
     std::shared_ptr<request_context> rctx,
     std::shared_ptr<statement_context> sctx
 ) noexcept:
+    id_(id_src_++),
     kind_(flat_task_kind::resolve),
     req_context_(std::move(rctx)),
     sctx_(std::move(sctx))
@@ -396,6 +407,7 @@ void flat_task::load() {
 
 flat_task::flat_task(task_enum_tag_t<flat_task_kind::load>, request_context* rctx,
     std::shared_ptr<executor::file::loader> ldr) noexcept:
+    id_(id_src_++),
     kind_(flat_task_kind::load),
     req_context_(rctx),
     loader_(std::move(ldr))
@@ -412,7 +424,4 @@ void flat_task::execute_wrapped() {
     }
 }
 
-}
-
-
-
+}  // namespace jogasaki::scheduler
diff --git a/src/jogasaki/scheduler/flat_task.h b/src/jogasaki/scheduler/flat_task.h
index 4268189d..d880d926 100644
--- a/src/jogasaki/scheduler/flat_task.h
+++ b/src/jogasaki/scheduler/flat_task.h
@@ -284,6 +284,7 @@ class cache_align flat_task {
     [[nodiscard]] request_context* req_context() const noexcept;
 
 private:
+    std::size_t id_{undefined_id};
     flat_task_kind kind_{};
     maybe_shared_ptr<request_context> req_context_{};
     std::shared_ptr<model::task> origin_{};
@@ -293,6 +294,8 @@ class cache_align flat_task {
     std::shared_ptr<statement_context> sctx_{};
     std::shared_ptr<executor::file::loader> loader_{};
 
+    static inline std::atomic_size_t id_src_{};  //NOLINT
+
     /**
      * @return true if job completes together with the task
      * @return false if only task completes
@@ -314,6 +317,7 @@ class cache_align flat_task {
     void execute_wrapped();
     void resubmit(request_context& req_context);
     void finish_job();
+    std::size_t assign_id();
 
     std::ostream& write_to(std::ostream& out) const {
         using namespace std::string_view_literals;