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

    https://github.com/apache/spark/pull/20057#discussion_r169025602
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/jdbc/PostgresDialect.scala ---
    @@ -85,15 +85,24 @@ private object PostgresDialect extends JdbcDialect {
         s"SELECT 1 FROM $table LIMIT 1"
       }
     
    +  override def isCascadingTruncateTable(): Option[Boolean] = Some(false)
    +
       /**
    -  * The SQL query used to truncate a table. For Postgres, the default 
behaviour is to
    -  * also truncate any descendant tables. As this is a (possibly unwanted) 
side-effect,
    -  * the Postgres dialect adds 'ONLY' to truncate only the table in question
    -  * @param table The name of the table.
    -  * @return The SQL query to use for truncating a table
    -  */
    -  override def getTruncateQuery(table: String): String = {
    -    s"TRUNCATE TABLE ONLY $table"
    +   * The SQL query used to truncate a table. For Postgres, the default 
behaviour is to
    +   * also truncate any descendant tables. As this is a (possibly unwanted) 
side-effect,
    +   * the Postgres dialect adds 'ONLY' to truncate only the table in 
question
    +   * @param table The table to truncate
    +   * @param cascade Whether or not to cascade the truncation. Default 
value is the value of
    +   *                isCascadingTruncateTable()
    +   * @return The SQL query to use for truncating a table
    +   */
    +  override def getTruncateQuery(
    +      table: String,
    +      cascade: Option[Boolean] = isCascadingTruncateTable): String = {
    +    cascade match {
    +      case Some(true) => s"TRUNCATE TABLE ONLY $table CASCADE"
    --- End diff --
    
    Done :-)


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