LSN external representation

Question

According to postgres documentation, 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. Does anyone know what is the meaning of these 2 hexadecimal numbers?

Answer 1

pg_lsn type supports - operation and returns byte offset, so here is the simplest way to convert from pg_lsn to bigint:

SELECT pg_current_wal_lsn() - '0/0'

For the example above it will look like this:

SELECT '16/374D848'::pg_lsn - '0/0'

Answer 2

Left pad the part right of the slash with zeros, then remove the slash, and you have the offset in bytes described by the documentation.