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

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cloud-fan 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_r389492481
 
 

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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:
   Good discussion here! I'm thinking about more cases:
   ```
   targetSize = 7
   lastButOneSize = 1
   lastSize = 7
   ```
   shall we merge the last partition into the previous partition?
   
   Maybe we can use a simple heuristic: merge 2 adjacent partitions if they don't exceed the target size too much (say 20%?). And we can apply it to partitions in the middle, not have to be the last 2 partitions.

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