当前事务日志的插入位置 和当前日志写入位置的区别？

pg_current_xlog_insert_location() text 获取当前事务日志的插入位置 ，指还在wal buffer中的位置。
pg_current_xlog_location() text 获取当前事务日志的写入位置，指已经调用了write后的位置（但是sync到磁盘之前），所以一定不在wal buffer了.




/*
 * Report the current WAL write location (same format as pg_start_backup etc)
 *
 * This is useful for determining how much of WAL is visible to an external
 * archiving process.  Note that the data before this point is written out
 * to the kernel, but is not necessarily synced to disk.
 */
Datum
pg_current_xlog_location(PG_FUNCTION_ARGS)
{
        XLogRecPtr      current_recptr;

        if (RecoveryInProgress())
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("recovery is in progress"),
                                 errhint("WAL control functions cannot be executed during recovery.")));

        current_recptr = GetXLogWriteRecPtr();

        PG_RETURN_LSN(current_recptr);
}

/*
 * Report the current WAL insert location (same format as pg_start_backup etc)
 *
 * This function is mostly for debugging purposes.
 */
Datum
pg_current_xlog_insert_location(PG_FUNCTION_ARGS)
{
        XLogRecPtr      current_recptr;

        if (RecoveryInProgress())
                ereport(ERROR,
                                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                                 errmsg("recovery is in progress"),
                                 errhint("WAL control functions cannot be executed during recovery.")));

        current_recptr = GetXLogInsertRecPtr();

        PG_RETURN_LSN(current_recptr);
}