如何查看主从复制的状态，且备库应用落后了多少字节
这些信息要在主库中查询
查看流复制的信息可以使用主库上的视图
select pid,state,client_addr,sync_priority,sync_state from pg_stat_replication;
pg_stat_replication中几个字断记录了发送wal的位置及备库接收到的wal的位置、
sent_location--发送wal的位置
write_location--备库接收到的wal的位置
flush_location—备库写wal日志到磁盘的位置
replay_location—备库应用日志的位置
查看备库落后主库多少字节
select pg_xlog_location_diff(pg_current_xlog_location(),replay_location)/1024/1024 as MB from pg_stat_replication; select pg_xlog_location_diff(pg_current_xlog_location(),replay_location)/1024/1024/1024 as GB from pg_stat_replication;
级联复制
select pg_xlog_location_diff(pg_last_xlog_replay_location(),replay_location)/1024/1024/1024 as GB from pg_stat_replication;
补充：pgsql之查看主备复制延迟
查看复制延迟：
10.0及以上：
SELECT pg_wal_lsn_diff(A .c1, replay_lsn) /(1024 * 1024) AS slave_latency_MB, pg_wal_lsn_diff(A .c1, sent_lsn) /(1024 * 1024) AS send_latency_MB, pg_wal_lsn_diff(A .c1, flush_lsn) /(1024 * 1024) AS flush_latency_MB, state, backend_start, now()::timestamp with time zone FROM pg_stat_replication, pg_current_wal_lsn() AS A(c1) WHERE client_addr='192.168.46.173' and application_name = 'standby1' ORDER BY slave_latency_MB, send_latency_MB DESC LIMIT 1;
注：
192.168.46.173 表示从库ip地址。
pg_wal_lsn_diff(lsn pg_lsn, lsn pg_lsn):计算两个预写式日志位置间的差别。
pg_current_wal_lsn()：获得当前预写式日志写入位置
小于10.0版本：
SELECT pg_xlog_location_diff(A .c1, replay_lsn) /(1024 * 1024) AS slave_latency_MB, pg_xlog_location_diff(A .c1, sent_lsn) /(1024 * 1024) AS send_latency_MB, pg_xlog_location_diff(A .c1, flush_lsn) /(1024 * 1024) AS flush_latency_MB, state, backend_start, now()::timestamp with time zone FROM pg_stat_replication, pg_current_xlog_location AS A(c1) WHERE client_addr='192.168.46.173' and application_name = 'standby1' ORDER BY slave_latency_MB, send_latency_MB DESC LIMIT 1;
注：
192.168.46.173 表示从库ip地址。
pg_xlog_location_diff(lsn pg_lsn, lsn pg_lsn):计算两个预写式日志位置间的差别。
pg_current_xlog_location ()：获得当前预写式日志写入位置
以上为个人经验，希望能给大家一个参考，也希望大家多多支持。如有错误或未考虑完全的地方，望不吝赐教。