Max/Min Expressions in Utils.scala

[oshritf]

Groupby with Max/Min throws null exception when column is other than IntegerType

More info:
scala/edu/berkeley/cs/rise/opaque/Utils.scala:

flatbuffersCreateField supports only creation of Null IntegerType (sets int value to 0)
Max and Min methods create Null Literal which currently not fully supported for all DataTypes and results in an exception.

Setting init value to zero (default for IntegerType currently) might not return valid results when data contains also negative values.

scala.MatchError: (null,LongType) (of class scala.Tuple2)
at edu.berkeley.cs.rise.opaque.Utils$.flatbuffersCreateField(Utils.scala:256)
origins - creation of Literal.create(null, child.dataType) in createInitialValuesVector

[ankurdave]

Thanks, we need to support serializing nulls of other types than IntegerType.

Setting init value to zero (default for IntegerType currently) might not return valid results when data contains also negative values.

I don't think this should occur for Max and Min, because the definitions of those aggregate expressions include null checks (e.g. for Max). The zero is only there as a placeholder.

Edit: Oops, never mind. The null check there is not sufficient since it only checks the nullity of the current max, not the new value. We need to fix that as well.

[ankurdave]

Moved the second part of this issue to #25.