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https://issues.apache.org/jira/browse/SPARK-25597?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16635430#comment-16635430
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Marco Gaido commented on SPARK-25597:
-------------------------------------
I think this is a duplicate of SPARK-25497. [~hkroger] may you please close it
accordingly? Thanks.
> SQL query with limit iterates the whole iterator when WholeStage code
> generation is enabled
> -------------------------------------------------------------------------------------------
>
> Key: SPARK-25597
> URL: https://issues.apache.org/jira/browse/SPARK-25597
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Affects Versions: 2.3.0
> Reporter: Hannu Kröger
> Priority: Major
>
> When _SELECT * FROM table LIMIT 1_ is executed, the WholeStageCodegenExec
> generates code that iterates the source iterator completely instead of just
> first element (per partition). This is a major performance penalty over the
> expected functionality. This does not happen if
> _spark.sql.codegen.wholeStage_ is set to false in spark context conf.
> In codegen mode {{LocalLimitExec}} overrides {{stopEarly}} method of
> {{BufferedRowIterator}} to stop consuming rows when enough rows are fetched.
> However, {{stopEarly}} is only called by code generated by {{InputAdapter}}
> and {{InputAdapter}} is not added to the plan for some reason.
> One potential way to fix this would be to check {{stopEarly}} in {{hasNext}}
> method of {{BufferedRowIterator}}. Interesting fact is that the {{next}} does
> not call {{hasNext}}. Therefore {{next}} might return empty result even if
> there is data to be fetched by the {{processNext}} method. Or add a check to
> stopEarly in shouldStop method.
> This commit might have broken the original functionality:
> [https://github.com/apache/spark/commit/b41ec997786e2be42a8a2a182212a610d08b221b]
> You can easily test with this:
> {code:java}
> // Scala test:
> test("iteration with limit") {
> sqlContext.read.format(
> classOf[infinite.DefaultSource].getPackage.getName)
> .schema("value INTEGER")
> .load()
> .createOrReplaceTempView("numbers")
> sqlContext.sql("SELECT * FROM numbers LIMIT 1").collect().length shouldBe
> 1
> }
> class DefaultSource extends SchemaRelationProvider {
> override def createRelation(sqlContext: SQLContext, parameters:
> Map[String, String], schema: StructType): BaseRelation = {
> new InfiniteRelation(sqlContext, schema)
> }
> }
> class InfiniteRDD(sc: SparkContext) extends RDD[Row](sc, Seq.empty) {
> override def compute(split: Partition, context: TaskContext):
> Iterator[Row] = {
> new PrintingIterator(Stream.from(0).map(i => Row(i)).iterator)
> }
> override protected def getPartitions: Array[Partition] = {
> Array(new Partition {
> override def index: Int = 0
> })
> }
> }
> class InfiniteRelation(override val sqlContext: SQLContext, val schema:
> StructType) extends BaseRelation with TableScan {
> override def buildScan(): RDD[Row] = new
> InfiniteRDD(sqlContext.sparkContext)
> }
> class PrintingIterator[T](backingIterator: Iterator[T]) extends
> AbstractIterator[T] {
> override def hasNext: Boolean = {
> backingIterator.hasNext
> }
> override def next(): T = {
> val nextValue = backingIterator.next()
> println(s"Printing Iterator is being called. Value: ${nextValue}")
> nextValue
> }
> }{code}
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