Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/22009#discussion_r224004348
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/v2/DataSourceV2Relation.scala
 ---
    @@ -169,15 +174,16 @@ object DataSourceV2Relation {
           options: Map[String, String],
           tableIdent: Option[TableIdentifier] = None,
           userSpecifiedSchema: Option[StructType] = None): 
DataSourceV2Relation = {
    -    val reader = source.createReader(options, userSpecifiedSchema)
    +    val readSupport = source.createReadSupport(options, 
userSpecifiedSchema)
    --- End diff --
    
    This is kind of a behavior change. Now when we append data to a data 
source, the data source must be readable, to provide a schema, which will be 
used to validate the input data schema.
    
    I don't have a strong feeling. Data source v2 is marked as involving so 
necessary behavior change is fine. cc @rdblue 


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