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https://issues.apache.org/jira/browse/SPARK-17728?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15535023#comment-15535023
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Herman van Hovell commented on SPARK-17728:
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I think calling {{explain(true)}} on your plans helps to understand what is
going on.
Spark executes the UDF 3x times because the optimizer collapses subsequent
projects (a project normally being much more expensive than a UDF call). In
your case the three projects get rewritten into one project, and the
expressions are rewritten in the following form:
{noformat}
structured_information -> fUdf('a)
plus_one -> fUdf('a).get("plusOne")
squared -> fUdf('a).get("squared")
{noformat}
It is a bit tricky to get around this, this might work:
{noformat}
val exploded = as
.withColumn("structured_information", explode(array(fUdf('a))))
.withColumn("plus_one", 'structured_information("plusOne"))
.withColumn("squared", 'structured_information("squared"))
{noformat}
> UDFs are run too many times
> ---------------------------
>
> Key: SPARK-17728
> URL: https://issues.apache.org/jira/browse/SPARK-17728
> Project: Spark
> Issue Type: Bug
> Components: Spark Core
> Affects Versions: 2.0.0
> Environment: Databricks Cloud / Spark 2.0.0
> Reporter: Jacob Eisinger
> Priority: Minor
> Attachments: over_optimized_udf.html
>
>
> h3. Background
> Llonger running processes that might run analytics or contact external
> services from UDFs. The response might not just be a field, but instead a
> structure of information. When attempting to break out this information, it
> is critical that query is optimized correctly.
> h3. Steps to Reproduce
> # Create some sample data.
> # Create a UDF that returns a multiple attributes.
> # Run UDF over some data.
> # Create new columns from the multiple attributes.
> # Observe run time.
> h3. Actual Results
> The UDF is executed *multiple times* _per row._
> h3. Expected Results
> The UDF should only be executed *once* _per row._
> h3. Workaround
> Cache the Dataset after UDF execution.
> h3. Details
> For code and more details, see [^over_optimized_udf.html]
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