文章中的函数与使用的pg10.3有差别，如下
pg_xlog_location_diff pg_wal_lsn_diff
pg_current_xlog_location pg_current_wal_lsn
pg_current_xlog_insert_location pg_current_wal_insert_lsn
链接地址：https://blog.2ndquadrant.com/basics-of-tuning-checkpoints/
