[GitHub] [spark] zhengruifeng commented on a diff in pull request #38612: [SPARK-41108][CONNECT] Control the max size of arrow batch

[GitBox <gi...@apache.org>]

zhengruifeng commented on code in PR #38612:
URL: https://github.com/apache/spark/pull/38612#discussion_r1019806473


##########
sql/core/src/main/scala/org/apache/spark/sql/execution/arrow/ArrowConverters.scala:
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@@ -161,17 +166,23 @@ private[sql] object ArrowConverters extends Logging {
         val writeChannel = new WriteChannel(Channels.newChannel(out))
 
         var rowCount = 0L
+        var estimatedBatchSize = arrowSchemaSize
         Utils.tryWithSafeFinally {
-          while (rowIter.hasNext && (maxRecordsPerBatch <= 0 || rowCount < maxRecordsPerBatch)) {
+          // Always write the schema.
+          MessageSerializer.serialize(writeChannel, arrowSchema)
+
+          // Always write the first row.
+          while (rowIter.hasNext && (rowCount == 0 || estimatedBatchSize < maxBatchSize)) {
             val row = rowIter.next()
             arrowWriter.write(row)
+            estimatedBatchSize += row.asInstanceOf[UnsafeRow].getSizeInBytes

Review Comment:
   refer to how the size is computed in [BroadcastExchange](https://github.com/apache/spark/blob/e17d8ecabcad6e84428752b977120ff355a4007a/sql/core/src/main/scala/org/apache/spark/sql/execution/exchange/BroadcastExchangeExec.scala#L150-L158) 
   
   but not 100% sure, should I use this instead?
   ```
   row match {
      case unsafe: UnsafeRow => estimatedBatchSize += unsafe.getSizeInBytes
      case _ => estimatedBatchSize += SizeEstimator.estimate(row)
   }
   ```
   
   cc @HyukjinKwon 



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