[GitHub] [spark] maryannxue commented on a change in pull request #27833: [SPARK-31070][SQL] make skew join split skewed partitions more evenly

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maryannxue commented on a change in pull request #27833: [SPARK-31070][SQL] make skew join split skewed partitions more evenly
URL: https://github.com/apache/spark/pull/27833#discussion_r389182488
 
 

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 File path: sql/core/src/main/scala/org/apache/spark/sql/execution/adaptive/OptimizeSkewedJoin.scala
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 @@ -115,18 +115,32 @@ case class OptimizeSkewedJoin(conf: SQLConf) extends Rule[SparkPlan] {
     partitionStartIndices += 0
     var i = 0
     var postMapPartitionSize = 0L
+    var lastPackagedPartitionSize = -1L
     while (i < mapPartitionSizes.length) {
       val nextMapPartitionSize = mapPartitionSizes(i)
       if (i > 0 && postMapPartitionSize + nextMapPartitionSize > targetSize) {
         partitionStartIndices += i
+        lastPackagedPartitionSize = postMapPartitionSize
         postMapPartitionSize = nextMapPartitionSize
       } else {
         postMapPartitionSize += nextMapPartitionSize
       }
       i += 1
     }
 
-    partitionStartIndices.toArray
+    if (lastPackagedPartitionSize > -1) {
+      val lastPartitionDiff = math.abs(targetSize - postMapPartitionSize)
+      val diffIfMergeLastPartition = math.abs(
+        lastPackagedPartitionSize + postMapPartitionSize - targetSize)
+      // If the last partition is very small, we should merge it to the previous partition.
+      if (lastPartitionDiff > diffIfMergeLastPartition * 2) {
 
 Review comment:
   I get your point here, but using target size and this formula doesn't quite make sense sometimes, e.g.,
   ```
   targetSize = 7
   lastButOneSize = 4
   lastSize = 4
   ```
   You'd get:
   ```
   diffLastPartition = 3
   diffLastTwoPartitions = 1
   ```
   This would satisfy your "very small" condition, yet I'd argue that first of all the last partition is well over half the target size, so I wouldn't consider it too small; second, what if previous partitions are mostly around size of 4 too? It would more even not to merge them, right?
   
   A simple way is probably get the average size of all partitions except the last one and merge if `(avgSize - lastSize) > (lastSize + lastButOneSize - avgSize)`. 

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