PostgreSQL 9.6.6 Documentation
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# 8.19. 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 XLOG. 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 positions.

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