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

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

HyukjinKwon commented on code in PR #38612:
URL: https://github.com/apache/spark/pull/38612#discussion_r1019838326


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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:
   The size of message should be based on Arrow but we are only able to know the size of the batch when Arrow batch is created.
   
   So I am fine with the current approach. I do believe that `UnsafeRow` has bigger size than `ArrowBatch` in general.
   
   One nit would be we should probably set the lower size in `maxBatchSize` to be conservative. For example, `maxBatchSize * 0.7`



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