[jira] [Resolved] (SPARK-25597) SQL query with limit iterates the whole iterator when WholeStage code generation is enabled

["Hannu Kröger (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SPARK-25597?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Hannu Kröger resolved SPARK-25597.
----------------------------------
    Resolution: Duplicate

> 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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