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From Muthu Jayakumar <bablo...@gmail.com>
Subject Re: Dataframe schema...
Date Thu, 20 Oct 2016 01:39:50 GMT
Hello Michael,

Thank you for looking into this query. In my case there seem to be an issue
when I union a parquet file read from disk versus another dataframe that I
construct in-memory. The only difference I see is the containsNull = true.
In fact, I do not see any errors with union on the simple schema of "col1
thru col4" above. But the problem seem to exist only on that
"some_histogram" column which contains the mixed containsNull = true/false.
Let me know if this helps.

Thanks,
Muthu



On Wed, Oct 19, 2016 at 6:21 PM, Michael Armbrust <michael@databricks.com>
wrote:

> Nullable is just a hint to the optimizer that its impossible for there to
> be a null value in this column, so that it can avoid generating code for
> null-checks.  When in doubt, we set nullable=true since it is always safer
> to check.
>
> Why in particular are you trying to change the nullability of the column?
>
> On Wed, Oct 19, 2016 at 6:07 PM, Muthu Jayakumar <babloo80@gmail.com>
> wrote:
>
>> Hello there,
>>
>> I am trying to understand how and when does DataFrame (or Dataset) sets
>> nullable = true vs false on a schema.
>>
>> Here is my observation from a sample code I tried...
>>
>>
>> scala> spark.createDataset(Seq((1, "a", 2.0d), (2, "b", 2.0d), (3, "c",
>> 2.0d))).toDF("col1", "col2", "col3").withColumn("col4",
>> lit("bla")).printSchema()
>> root
>>  |-- col1: integer (nullable = false)
>>  |-- col2: string (nullable = true)
>>  |-- col3: double (nullable = false)
>>  |-- col4: string (nullable = false)
>>
>>
>> scala> spark.createDataset(Seq((1, "a", 2.0d), (2, "b", 2.0d), (3, "c",
>> 2.0d))).toDF("col1", "col2", "col3").withColumn("col4",
>> lit("bla")).write.parquet("/tmp/sample.parquet")
>>
>> scala> spark.read.parquet("/tmp/sample.parquet").printSchema()
>> root
>>  |-- col1: integer (nullable = true)
>>  |-- col2: string (nullable = true)
>>  |-- col3: double (nullable = true)
>>  |-- col4: string (nullable = true)
>>
>>
>> The place where this seem to get me into trouble is when I try to union
>> one data-structure from in-memory (notice that in the below schema the
>> highlighted element is represented as 'false' for in-memory created schema)
>> and one from file that starts out with a schema like below...
>>
>>  |-- some_histogram: struct (nullable = true)
>>  |    |-- values: array (nullable = true)
>>  |    |    |-- element: double (containsNull = true)
>>  |    |-- freq: array (nullable = true)
>>  |    |    |-- element: long (containsNull = true)
>>
>> Is there a way to convert this attribute from true to false without
>> running any mapping / udf on that column?
>>
>> Please advice,
>> Muthu
>>
>
>

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