fix inaccurate db_time.

src/sql/engine/ob_operator.h

@@ -501,7 +501,6 @@ public:
   ObBatchRows &get_brs() { return brs_; }
   // Drain exchange in data for PX, or producer DFO will be blocked.
   virtual int drain_exch();
-
   void set_pushdown_param_null(const common::ObIArray<ObDynamicParamSetter> &rescan_params);
   void set_feedback_node_idx(int64_t idx)
   { fb_node_idx_ = idx; }
@@ -522,6 +521,7 @@ protected:
   // try open operator
   int try_open() { return opened_ ? common::OB_SUCCESS : open(); }
 
+
   virtual void do_clear_datum_eval_flag();
   void clear_batch_end_flag() { brs_.end_ = false; }
   inline void reset_batchrows()
@@ -571,6 +571,9 @@ private:
   int output_expr_sanity_check();
   int output_expr_sanity_check_batch();
   int setup_op_feedback_info();
+  int do_drain_exch();
+  // child can implement this interface, but can't call this directly
+  virtual int inner_drain_exch() { return common::OB_SUCCESS; };
 protected:
   const ObOpSpec &spec_;
   ObExecContext &ctx_;