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https://issues.apache.org/jira/browse/SPARK-15642?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Christian Zommerfelds updated SPARK-15642:
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Affects Version/s: 2.2.0
> Metadata gets lost when selecting a field of a StructType
> ---------------------------------------------------------
>
> Key: SPARK-15642
> URL: https://issues.apache.org/jira/browse/SPARK-15642
> Project: Spark
> Issue Type: Bug
> Components: SQL
> Affects Versions: 1.6.0, 1.6.1, 2.2.0
> Reporter: Christian Zommerfelds
>
> Hi,
> When working with Data Frames, sometimes I find myself needing to write a
> function that creates multiple columns. Since that is not directly possible,
> I create a function that returns a StructType, and then call select() to
> assign the fields to different columns. However, I noticed that the metadata
> gets lost when I do that.
> Example: (Python)
> {code}
> In: schema = StructType([StructField('foo', StructType([
> StructField('features', ArrayType(IntegerType())),
> StructField('label', DoubleType(), False,
> {'ml_attr': {'type': 'nominal', 'vals': ['0.0', '1.0']}}
> )
> ]))])
> In: df = sqlContext.createDataFrame([Row(foo=Row(features=[1,2], label=0.0)),
> Row(foo=Row(features=[3,4], label=1.0))], schema)
> In: df.schema.fields[0].dataType.fields[1].metadata
> Out: {'ml_attr': {'type': 'nominal', 'vals': ['0.0', '1.0']}}
> In: df2 = df.select(df.foo['label'])
> In: df2.schema.fields[0].metadata
> Out: {}
> {code}
> Expected: same metadata (ml_attrib...)
> My work around is to create a new Data Frame from RDD, because as far as I
> know PySpark doesn't support adding metadata once the DF is created (should I
> create another issue for that?). Work around example:
> {code}
> In: df3 = sqlContext.createDataFrame(df2.rdd,
> StructType([schema.fields[0].dataType.fields[1]]))
> In: df3.schema.fields[0].metadata
> Out: {'ml_attr': {'type': 'nominal', 'vals': ['0.0', '1.0']}}
> {code}
> I am not sure if this affects the Scala API. (EDIT: yes it does. See test
> case at https://github.com/apache/spark/pull/13467/files)
> Let me know if I can provide any other information.
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