Github user dongjoon-hyun commented on a diff in the pull request:

    https://github.com/apache/spark/pull/14086#discussion_r71471006
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/DataFrameWriter.scala ---
    @@ -419,8 +422,13 @@ final class DataFrameWriter[T] private[sql](ds: 
Dataset[T]) {
           }
     
           if (mode == SaveMode.Overwrite && tableExists) {
    -        JdbcUtils.dropTable(conn, table)
    -        tableExists = false
    +        if (extraOptions.getOrElse("truncate", "false").toBoolean &&
    +            JdbcUtils.isCascadingTruncateTable(url) == Some(false)) {
    +          JdbcUtils.truncateTable(conn, table)
    --- End diff --
    
    For my understanding, I will ask one question. 
    
    Literally, we should not do whatever we do with drop, e.g., we should not 
drop INDEX, right?


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