Question about type cast in ExpressionReducer's reduce procedure

[郭健]

Hi all,
            I am implementing a STR_TO_DATE scalar SQL function to flink, and 
found return type casted from java.sql.Date to Integer in Flink’s 
ExpressionReducer：
https://github.com/apache/flink/blob/master/flink-libraries/flink-table/src/main/scala/org/apache/flink/table/codegen/ExpressionReducer.scala#L56

// we need to cast here for RexBuilder.makeLiteral


      case (SqlTypeName.DATE, e) =>


        Some(


          
rexBuilder.makeCast(typeFactory.createTypeFromTypeInfo(BasicTypeInfo.INT_TYPE_INFO),
 e)


        )



            so str_to_date('01,5,2013','%d,%m,%Y')" must return an Integer, 
which conflicted with my implementation.

            My question is: why should we do this? I have seen in comments the 
reason to do this here is: “we need to cast here for RexBuilder.makeLiteral”, 
But is it reasonale to change user function’s return Type? Should we restore 
the origin return type after the reduce?


Thanks,
Aegeaner