Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/9215#discussion_r42929484
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/metric/SQLMetrics.scala 
---
    @@ -62,7 +67,19 @@ private[sql] trait SQLMetricValue[T] extends 
Serializable {
     private[sql] class LongSQLMetricValue(private var _value : Long) extends 
SQLMetricValue[Long] {
     
       def add(incr: Long): LongSQLMetricValue = {
    -    _value += incr
    +    // Some LongSQLMetric will use -1 as initial value, so if the 
accumulator is never updated,
    +    // we can filter it out later.  However, when `add` is called, the 
accumulator is valid, we
    +    // should turn -1 to 0.
    +    if (_value < 0) {
    +      _value = 0
    +    }
    +
    +    // Some LongSQLMetric will use -1 as initial value, when we merge 
accumulator updates at driver
    +    // side, we should ignore these -1 values.
    +    if (incr > 0) {
    +      _value += incr
    +    }
    +
    --- End diff --
    
    I was trying to reuse current codebase and didn't create a new 
SQLMetric(including new MetricValue, MetricParam, etc.). Is it worth to create 
a new one so that we won't hurt performance for numRows?


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