oracle structure of a row

Structure of a Row,以下圖:

     Row data is stored in database blocks as variable-length records,Columns for a row are generally

 stored in the order in which they are defined and any trailing NULL columns are not stored.

    Note: A single byte for column length is required for non trailing NULL columns,each row in a table has:

    一、Row header: Used to stored the number of columns in the row,the chaining information,

           and the lock status.

    二、 Row data: For each column,the oracle server stores the column length and value

           (one byte is needed to store the column length if the column will require more than

            250 bytes of storage in which case three bytes will be used for column length.

            the column value is stored immediately following the column length bytes.)

      Adjacent rows do not need any space between them,

     each row in the block has a slot in the row directory.

     the directory slot points to the begining of the row.