Sorry for the late response. Here is what I am seeing...


Schema from parquet file.
d1.printSchema()
root
 |-- task_id: string (nullable = true)
 |-- task_name: string (nullable = true)
 |-- some_histogram: struct (nullable = true)
 |    |-- values: array (nullable = true)
 |    |    |-- element: double (containsNull = true)
 |    |-- freq: array (nullable = true)
 |    |    |-- element: long (containsNull = true)

d2.printSchema() //Data created using dataframe and/or processed before writing to parquet file.
root
 |-- task_id: string (nullable = true)
 |-- task_name: string (nullable = true)
 |-- some_histogram: struct (nullable = true)
 |    |-- values: array (nullable = true)
 |    |    |-- element: double (containsNull = false)
 |    |-- freq: array (nullable = true)
 |    |    |-- element: long (containsNull = false)

d1.union(d2).printSchema()
Exception in thread "main" org.apache.spark.sql.AnalysisException: unresolved operator 'Union;
at org.apache.spark.sql.catalyst.analysis.CheckAnalysis$class.failAnalysis(CheckAnalysis.scala:40)
at org.apache.spark.sql.catalyst.analysis.Analyzer.failAnalysis(Analyzer.scala:58)
at org.apache.spark.sql.catalyst.analysis.CheckAnalysis$$anonfun$checkAnalysis$1.apply(CheckAnalysis.scala:361)
at org.apache.spark.sql.catalyst.analysis.CheckAnalysis$$anonfun$checkAnalysis$1.apply(CheckAnalysis.scala:67)
at org.apache.spark.sql.catalyst.trees.TreeNode.foreachUp(TreeNode.scala:126)
at org.apache.spark.sql.catalyst.analysis.CheckAnalysis$class.checkAnalysis(CheckAnalysis.scala:67)
at org.apache.spark.sql.catalyst.analysis.Analyzer.checkAnalysis(Analyzer.scala:58)
at org.apache.spark.sql.execution.QueryExecution.assertAnalyzed(QueryExecution.scala:49)
at org.apache.spark.sql.Dataset.<init>(Dataset.scala:161)
at org.apache.spark.sql.Dataset.<init>(Dataset.scala:167)
at org.apache.spark.sql.Dataset$.apply(Dataset.scala:59)
at org.apache.spark.sql.Dataset.withTypedPlan(Dataset.scala:2594)
at org.apache.spark.sql.Dataset.union(Dataset.scala:1459)

Please advice,
Muthu

On Thu, Oct 20, 2016 at 1:46 AM, Michael Armbrust <michael@databricks.com> wrote:
What is the issue you see when unioning?

On Wed, Oct 19, 2016 at 6:39 PM, Muthu Jayakumar <babloo80@gmail.com> wrote:
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