append data using saveAsTable should adjust the data types

Seq(1 -> "a").toDF("i", "j").write.saveAsTable("t")

Seq("c" -> 3).toDF("i", "j").write.mode("append").saveAsTable("t")

scala> sql("select * from t").show

org.apache.spark.SparkException: Job aborted due to stage failure: Task 1 in stage 10.0 failed 1 times, most recent failure: Lost task 1.0 in stage 10.0 (TID 15, localhost, executor driver): java.lang.UnsupportedOperationException: Unimplemented type: IntegerType
 at org.apache.spark.sql.execution.datasources.parquet.VectorizedColumnReader.readBinaryBatch(VectorizedColumnReader.java:473)
 at org.apache.spark.sql.execution.datasources.parquet.VectorizedColumnReader.readBatch(VectorizedColumnReader.java:214)
 at org.apache.spark.sql.execution.datasources.parquet.VectorizedParquetRecordReader.nextBatch(VectorizedParquetRecordReader.java:261)
...

scala> sql("select * from tx").show
+----+---+
|   i|  j|
+----+---+
|null|  3|
|   1|  a|
+----+---+