Optimize the merging of operator stats

Author: shangm2

presto-main/src/main/java/com/facebook/presto/execution/StageExecutionInfo.java

@@ -73,11 +73,8 @@ public static StageExecutionInfo create(
     {
         TaskStatsAggregator taskStatsAggregator = new TaskStatsAggregator();
         taskStatsAggregator.increaseTotalTaskCount(taskInfos.size());
-
         taskStatsAggregator.mergeRuntimeStats(stageRuntimeStats);
 
-        List<TaskStats> allTaskStats = new ArrayList<>();
-
         for (TaskInfo taskInfo : taskInfos) {
             TaskState taskState = taskInfo.getTaskStatus().getState();
             if (taskState.isDone()) {
@@ -88,7 +85,6 @@ public static StageExecutionInfo create(
             }
 
             TaskStats taskStats = taskInfo.getStats();
-            allTaskStats.add(taskStats);
 
             if (state == FINISHED && taskInfo.getTaskStatus().getState() == TaskState.FAILED) {
                 taskStatsAggregator.increaseRetriedCpuTime(taskStats.getTotalCpuTimeInNanos());
@@ -100,9 +96,6 @@ public static StageExecutionInfo create(
             }
 
             taskStatsAggregator.increaseBufferedDataSize(taskInfo.getOutputBuffers().getTotalBufferedBytes());
-        }
-
-        for (TaskStats taskStats : allTaskStats) {
             taskStatsAggregator.processTaskStats(taskStats);
         }
 
@@ -221,13 +214,13 @@ public static StageExecutionInfo unscheduledExecutionInfo(int stageId, boolean i
     private static class TaskStatsAggregator
     {
         private int totalTasks;
-        int runningTasks = 0;
-        int completedTasks = 0;
-        long retriedCpuTime = 0;
-        long bufferedDataSize = 0;
+        private int runningTasks;
+        private int completedTasks;
+        private long retriedCpuTime;
+        private long bufferedDataSize;
 
-        boolean fullyBlocked = true;
-        Set<BlockedReason> blockedReasons = new HashSet<>();
+        private boolean fullyBlocked = true;
+        private final Set<BlockedReason> blockedReasons = new HashSet<>();
 
         private int totalDrivers;
         private int queuedDrivers;
@@ -263,8 +256,8 @@ private static class TaskStatsAggregator
         private int maxFullGcSec;
         private int totalFullGcSec;
 
-        Map<String, OperatorStats> operatorToStats = new HashMap<>();
-        RuntimeStats mergedRuntimeStats = new RuntimeStats();
+        private final Map<String, OperatorStats> operatorToStats = new HashMap<>();
+        private final RuntimeStats mergedRuntimeStats = new RuntimeStats();
 
         public void processTaskStats(TaskStats taskStats)
         {