PG流复制lag解析

postgres=# select * from pg_stat_replication;
-[ RECORD 1 ]----+------------------------------
pid              | 31700
usesysid         | 16384
usename          | repl
application_name | walreceiver
client_addr      | 192.168.1.72
client_hostname  | 
client_port      | 43210
backend_start    | 2022-09-23 20:53:39.313239+08
backend_xmin     | 
state            | streaming
sent_lsn         | 2/80000000
write_lsn        | 2/80000000
flush_lsn        | 2/80000000
replay_lsn       | 2/80000000
write_lag        | 
flush_lag        | 
replay_lag       | 
sync_priority    | 0
sync_state       | async
reply_time       | 2022-09-23 20:53:59.57744+08

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关于write_lag,flush_lag,replay_lag所指的范围，另外箭头尖的指向分别为send_lsn,write_lsn,flush_lsn,replay_lsn