XLogRecPtr is a data type representing a pointer to a location in the current WAL file.
As of PostgreSQL 9.3, XLogRecPtr is an unsigned 64 bit integer, defined in include/access/xlogdefs.h:
/* * Pointer to a location in the XLOG. These pointers are 64 bits wide, * because we don't want them ever to overflow. */ typedef uint64 XLogRecPtr;
In PostgreSQL 9.2 and earlier, XLogRecPtr was a struct consisting of two 32 bit integers:
typedef struct XLogRecPtr
{
uint32 xlogid; /* log file #, 0 based */
uint32 xrecoff; /* byte offset of location in log file */
} XLogRecPtr;
Externally XLogRecPtr is displayed as an LSN consisting of two sets of up to 8 hexadecimal digits separated by a slash, e.g.
postgres=# SELECT pg_last_wal_receive_lsn(); pg_last_wal_receive_lsn ------------------------- 0/40043D8 (1 row)
Beginning with PostgreSQL 9.4, a discrete datatype pg_lsn was introduced to represent LSNs.
An invalid XLogRecPtr is represented in 9.3 or later as zero (constant InvalidXLogRecPtr); in 9.2 and earlier the xrecoff value is set to zero. In both cases the macro XLogRecPtrIsInvalid() can be used to check the validity of an XLogRecPtr.