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https://issues.apache.org/jira/browse/SPARK-18748?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16845559#comment-16845559
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Ohad Raviv commented on SPARK-18748:
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[~kelemen] - thanks for sharing.
> UDF multiple evaluations causes very poor performance
> -----------------------------------------------------
>
> Key: SPARK-18748
> URL: https://issues.apache.org/jira/browse/SPARK-18748
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Affects Versions: 2.3.0, 2.4.0
> Reporter: Ohad Raviv
> Priority: Major
>
> We have a use case where we have a relatively expensive UDF that needs to be
> calculated. The problem is that instead of being calculated once, it gets
> calculated over and over again.
> for example:
> {quote}
> def veryExpensiveCalc(str:String) = \{println("blahblah1"); "nothing"\}
> hiveContext.udf.register("veryExpensiveCalc", veryExpensiveCalc _)
> hiveContext.sql("select * from (select veryExpensiveCalc('a') c)z where c is
> not null and c<>''").show
> {quote}
> with the output:
> {quote}
> blahblah1
> blahblah1
> blahblah1
> +-------+
> | c|
> +-------+
> |nothing|
> +-------+
> {quote}
> You can see that for each reference of column "c" you will get the println.
> that causes very poor performance for our real use case.
> This also came out on StackOverflow:
> http://stackoverflow.com/questions/40320563/spark-udf-called-more-than-once-per-record-when-df-has-too-many-columns
> http://stackoverflow.com/questions/34587596/trying-to-turn-a-blob-into-multiple-columns-in-spark/
> with two problematic work-arounds:
> 1. cache() after the first time. e.g.
> {quote}
> hiveContext.sql("select veryExpensiveCalc('a') as c").cache().where("c is not
> null and c<>''").show
> {quote}
> while it works, in our case we can't do that because the table is too big to
> cache.
> 2. move back and forth to rdd:
> {quote}
> val df = hiveContext.sql("select veryExpensiveCalc('a') as c")
> hiveContext.createDataFrame(df.rdd, df.schema).where("c is not null and
> c<>''").show
> {quote}
> which works but then we loose some of the optimizations like push down
> predicate features, etc. and its very ugly.
> Any ideas on how we can make the UDF get calculated just once in a reasonable
> way?
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