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.