pg_lsn Type
    The pg_lsn data type can be used to store LSN (Log Sequence
    Number) data which is a pointer to a location in the WAL. This type is a
    representation of XLogRecPtr and an internal system type of
    PostgreSQL.
   
    Internally, an LSN is a 64-bit integer, representing a byte position in
    the write-ahead log stream.  It is printed as two hexadecimal numbers of
    up to 8 digits each, separated by a slash; for example,
    16/B374D848.  The pg_lsn type supports the
    standard comparison operators, like = and
    >.  Two LSNs can be subtracted using the
    - operator; the result is the number of bytes separating
    those write-ahead log locations.  Also the number of bytes can be
    added into and subtracted from LSN using the
    +(pg_lsn,numeric) and
    -(pg_lsn,numeric) operators, respectively. Note that
    the calculated LSN should be in the range of pg_lsn type,
    i.e., between 0/0 and
    FFFFFFFF/FFFFFFFF.