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https://issues.apache.org/jira/browse/SPARK-34644?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17296556#comment-17296556
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Tanel Kiis commented on SPARK-34644:
------------------------------------
UDF with internal state should be marked as non-deterministic with the
`asNondeterministic()` method on it.
If the issue persists after that, then it should be considered a bug.
> UDF returning array followed by explode calls the UDF multiple times and
> could return wrong results
> ---------------------------------------------------------------------------------------------------
>
> Key: SPARK-34644
> URL: https://issues.apache.org/jira/browse/SPARK-34644
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Affects Versions: 3.1.1
> Reporter: Gavrilescu Laurentiu
> Priority: Major
>
> *Applying an UDF followed by explode calls the UDF multiple times.*
> Using *persist* after applying the UDF mitigates the problem
> Consider the following code to reproduce it:
> {code:java}
> object Bug {
> def main(args: Array[String]) {
> val sparkSession: SparkSession =
> SparkSession.builder.master("local[4]").getOrCreate()
> val invocations = sparkSession.sparkContext.longAccumulator("invocations")
> def showTiming[T](body: => T): T = {
> val t0 = System.nanoTime()
> invocations.reset()
> val res = body
> val t1 = System.nanoTime()
> println(s"invocations=${invocations.value}, time=${(t1 - t0) / 1e9}")
> res
> }
> def expensive(n: Int) = {
> Thread.sleep(100)
> invocations.add(1)
> 1
> }
> val expensiveUdf = udf((x: Int) => (1 to 10) map { _ => expensive(x) })
> val df = sparkSession.range(10).toDF()
> showTiming(df
> .select(explode(expensiveUdf(col("id"))))
> .select(sum(col("col")))
> .show())
> showTiming(df.select(expensiveUdf(col("id")).as("values"))
> .persist()
> .select(explode(col("values")))
> .select(sum("col"))
> .show())
> }
> }
> {code}
> =>
> {code:java}
> first: invocations=300, time=11.342076635
> second: invocations=100, time=3.351682967{code}
> This can have undesired behavior and can return wrong results if a managed
> state is used inside the UDF.
> Imagine having the following scenario:
> 1. you have a dataframe with some string columns
> 2. you have an expensive function that creates a score based on some string
> input
> 3. you want to get all the distinct values from all the columns and their
> score - there is an executor level cache that holds the score values for
> strings to minimize the execution of the expensive function
> consider the following code to reproduce it:
> {code:java}
> case class RowWithStrings(c1: String, c2: String, c3: String)
> case class ValueScore(value: String, score: Double)
> object Bug {
> val columns: List[String] = List("c1", "c2", "c3")
> def score(input: String): Double = {
> // insert expensive function here
> input.toDouble
> }
> def main(args: Array[String]) {
> lazy val sparkSession: SparkSession = {
> val sparkSession = SparkSession.builder.master("local[4]")
> .getOrCreate()
> sparkSession
> }
> // some cache over expensive operation
> val cache: TrieMap[String, Double] = TrieMap[String, Double]()
> // get scores for all columns in the row
> val body = (row: Row) => {
> val arr = ArrayBuffer[ValueScore]()
> columns foreach {
> column =>
> val value = row.getAs[String](column)
> if (!cache.contains(value)) {
> val computedScore = score(value)
> arr += ValueScore(value, computedScore)
> cache(value) = computedScore
> }
> }
> arr
> }
> val basicUdf = udf(body)
> val values = (1 to 5) map {
> idx =>
> // repeated values
> RowWithStrings(idx.toString, idx.toString, idx.toString)
> }
> import sparkSession.implicits._
> val df = values.toDF("c1", "c2", "c3").persist()
> val allCols = df.columns.map(col)
> df.select(basicUdf(struct(allCols: _*)).as("valuesScore"))
> .select(explode(col("valuesScore")))
> .distinct()
> .show()
> }
> }
> {code}
> this shows:
> {code:java}
> +---+
> |col|
> +---+
> +---+
> {code}
> When adding persist before explode, the result is correct:
> {code:java}
> df.select(basicUdf(struct(allCols: _*)).as("valuesScore"))
> .persist()
> .select(explode(col("valuesScore")))
> .distinct()
> .show()
> {code}
> =>
> {code:java}
> +--------+
> | col|
> +--------+
> |{2, 2.0}|
> |{4, 4.0}|
> |{3, 3.0}|
> |{5, 5.0}|
> |{1, 1.0}|
> +--------+
> {code}
> This is not reproducible using 3.0.2 version.
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