[GitHub] [spark] maropu commented on a change in pull request #22347: [SPARK-25353][SQL] executeTake in SparkPlan is modified to avoid unnecessary decoding.

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maropu commented on a change in pull request #22347: [SPARK-25353][SQL] executeTake in SparkPlan is modified to avoid unnecessary decoding.
URL: https://github.com/apache/spark/pull/22347#discussion_r298857999
 
 

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 File path: sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala
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 @@ -348,30 +349,30 @@ abstract class SparkPlan extends QueryPlan[SparkPlan] with Logging with Serializ
         // Otherwise, interpolate the number of partitions we need to try, but overestimate
         // it by 50%. We also cap the estimation in the end.
         val limitScaleUpFactor = Math.max(sqlContext.conf.limitScaleUpFactor, 2)
-        if (buf.isEmpty) {
+        if (scannedRowCount == 0) {
           numPartsToTry = partsScanned * limitScaleUpFactor
         } else {
-          val left = n - buf.size
+          val left = n - scannedRowCount
           // As left > 0, numPartsToTry is always >= 1
-          numPartsToTry = Math.ceil(1.5 * left * partsScanned / buf.size).toInt
+          numPartsToTry = Math.ceil(1.5 * left * partsScanned / scannedRowCount).toInt
           numPartsToTry = Math.min(numPartsToTry, partsScanned * limitScaleUpFactor)
         }
       }
 
       val p = partsScanned.until(math.min(partsScanned + numPartsToTry, totalParts).toInt)
       val sc = sqlContext.sparkContext
-      val res = sc.runJob(childRDD,
-        (it: Iterator[Array[Byte]]) => if (it.hasNext) it.next() else Array.empty[Byte], p)
-
-      buf ++= res.flatMap(decodeUnsafeRows)
+      val res = sc.runJob(childRDD, (it: Iterator[(Long, Array[Byte])]) =>
+        if (it.hasNext) it.next() else (0L, Array.empty[Byte]), p)
 
+      buf ++= res.map(_._2)
+      scannedRowCount += res.map(_._1).sum
       partsScanned += p.size
     }
 
-    if (buf.size > n) {
-      buf.take(n).toArray
+    if (scannedRowCount > n) {
+      buf.iterator.flatMap(decodeUnsafeRows).take(n).toArray
 
 Review comment:
   nit: can you put some comments about why we need `.iterator` here? we need it to avoid unnecessary decoding, right?

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