srowen commented on a change in pull request #26809: [SPARK-30185][SQL]
Implement Dataset.tail API
URL: https://github.com/apache/spark/pull/26809#discussion_r357131688
##########
File path:
sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala
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@@ -426,23 +456,46 @@ abstract class SparkPlan extends QueryPlan[SparkPlan]
with Logging with Serializ
}
}
- val p = partsScanned.until(math.min(partsScanned + numPartsToTry,
totalParts).toInt)
+ val parts = partsScanned.until(math.min(partsScanned + numPartsToTry,
totalParts).toInt)
+ val partsToScan = if (reverse) {
+ // Reverse partitions to scan. So, if parts was [1, 2, 3] in 200
partitions (0 to 199),
+ // it becomes [198, 197, 196].
+ parts.map(p => (totalParts - 1) - p)
+ } else {
+ parts
+ }
val sc = sqlContext.sparkContext
val res = sc.runJob(childRDD, (it: Iterator[(Long, Array[Byte])]) =>
- if (it.hasNext) it.next() else (0L, Array.empty[Byte]), p)
+ if (it.hasNext) it.next() else (0L, Array.empty[Byte]), partsToScan)
var i = 0
- while (buf.length < n && i < res.length) {
- val rows = decodeUnsafeRows(res(i)._2)
- val rowsToTake = if (n - buf.length >= res(i)._1) {
- rows.toArray
- } else {
- rows.take(n - buf.length).toArray
+
+ if (reverse) {
+ while (buf.length < n && i < res.length) {
+ val rows = decodeUnsafeRows(res(i)._2)
+ if (n - buf.length >= res(i)._1) {
+ buf.insertAll(0, rows.toArray[InternalRow])
Review comment:
OK I get it now. I think this is correct. You are reading partitions in
reverse order, and prepending their contents in order. This gives you the tail
in order, yes.
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