Github user fhueske commented on a diff in the pull request:

    https://github.com/apache/flink/pull/3166#discussion_r102507787
  
    --- Diff: 
flink-libraries/flink-table/src/test/scala/org/apache/flink/table/TableSourceTest.scala
 ---
    @@ -126,21 +156,49 @@ class TableSourceTest extends TableTestBase {
     
       @Test
       def testStreamProjectableSourceScanNoIdentityCalc(): Unit = {
    -    val (csvTable, tableName) = tableSource
    +    val (tableSource, tableName) = csvTable
         val util = streamTestUtil()
         val tEnv = util.tEnv
     
    -    tEnv.registerTableSource(tableName, csvTable)
    +    tEnv.registerTableSource(tableName, tableSource)
     
         val result = tEnv
           .scan(tableName)
           .select('id, 'score, 'first)
     
    -    val expected = sourceStreamTableNode(tableName, noCalcFields)
    +    val expected = projectableSourceStreamTableNode(tableName, 
noCalcFields)
         util.verifyTable(result, expected)
       }
     
       @Test
    +  def testStreamFilterableSourceScanPlanTableApi(): Unit = {
    +    val (tableSource, tableName) = filterableTableSource
    +    val util = streamTestUtil()
    +    val tEnv = util.tEnv
    +
    +    tEnv.registerTableSource(tableName, tableSource)
    +
    +    val result = tEnv
    +      .scan(tableName)
    +      .select('price, 'id, 'amount)
    +      .where("amount > 2 && price * 2 < 32")
    --- End diff --
    
    An example for an unsupported predicate would be 
`'id.cast(BasicTypeInfo.STRING_TYPE_INFO) === "abc"`. This throws and exception 
when translating it to an `Expression`.
    
    As said before, unsupported expressions should be gracefully handled by not 
failing but instead by not offering this `RexNode` to the 
`FilterableTableSource` and evaluating it in the `DataSetCalc`.
    I would suggest to use `CAST` as an example to implement the graceful 
handling and adding support for it once the failure-free translation works. 


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