在备库上监控 WAL 接收和回放延迟（pg_stat_wal_receiver）

命令

-- Run on the standby:
SELECT pg_is_in_recovery() AS in_recovery,
       pg_last_wal_receive_lsn() AS receive_lsn,
       pg_last_wal_replay_lsn() AS replay_lsn,
       pg_size_pretty(pg_wal_lsn_diff(pg_last_wal_receive_lsn(), pg_last_wal_replay_lsn())) AS replay_backlog,
       now() - pg_last_xact_replay_timestamp() AS last_replay_age,
       pg_get_wal_replay_pause_state() AS pause_state;
SELECT pid, status, sender_host, sender_port, slot_name, received_tli, flushed_lsn, latest_end_lsn, now() - last_msg_receipt_time AS since_last_msg FROM pg_stat_wal_receiver;

作用

显示备库已接收多少 WAL、已应用多少（回放积压）、最后回放事务的时长、回放是否暂停，以及 walreceiver 连接状态（哪个主库、复制槽、timeline、最后消息时间）。

适用场景

用于在主库不可达或级联备库上诊断延迟；出现 'standby behind' 告警时，区分网络（接收）问题和回放（I/O、冲突）问题。

注意

如果 pg_stat_wal_receiver 不返回任何行，说明备库没有进行流复制（从归档获取或已断开）。即使主库没有写入，last_replay_age 也会增长；请结合 receive_lsn/replay_lsn 的差值来解读。如果接收在推进而回放积压在增长，说明备库上有长查询（max_standby_streaming_delay）或存在 I/O 瓶颈。