Github user viirya commented on a diff in the pull request:
https://github.com/apache/spark/pull/16696#discussion_r98002931
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/plans/logical/basicLogicalOperators.scala
---
@@ -727,37 +728,18 @@ case class GlobalLimit(limitExpr: Expression, child:
LogicalPlan) extends UnaryN
}
override def computeStats(conf: CatalystConf): Statistics = {
val limit = limitExpr.eval().asInstanceOf[Int]
- val sizeInBytes = if (limit == 0) {
- // sizeInBytes can't be zero, or sizeInBytes of BinaryNode will also
be zero
- // (product of children).
- 1
- } else {
- (limit: Long) * output.map(a => a.dataType.defaultSize).sum
- }
- child.stats(conf).copy(sizeInBytes = sizeInBytes)
+ val childStats = child.stats(conf)
+ // Don't propagate column stats, because we don't know the
distribution after a limit operation
+ Statistics(
+ sizeInBytes = EstimationUtils.getOutputSize(output, limit,
childStats.attributeStats),
+ rowCount = Some(limit),
--- End diff --
Actually the `rowCount` for `LocalLimit` and `GlobalLimit` should be
different. For `LocalLimit`, `limit` is just the row count for one partition.
But we can't get the number of partitions here, I think. As the actual row
number might be quite bigger than the `limit`, maybe we should set it as None.
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