Identify Replication delay/lag on Streaming replication

As specified prior, sync between Primary and Standby servers can undoubtedly be seen by executing below functions,

On Primary:
SELECT pg_current_xlog_location();

On Standby:
SELECT pg_last_xlog_receive_location();
SELECT pg_last_xlog_replay_location();

furthermore, on a real-traffic extensive database wherein hits after hits onto database is expected and resulting in series of writes/selects will impact on above Hexa-decimal values being represent the state of sync between Primary and Standby Servers.

To Check replication status, use below function by firing this on Primary Node:

PG_STAT_REPLICATION:

Column	Type	Description
pid	integer	Process ID of a WAL sender process
usesysid	oid	OID of the user logged into this WAL sender process
usename	name	Name of the user logged into this WAL sender process
application_name	text	Name of the application that is connected to this WAL sender
client_addr	inet	IP address of the client connected to this WAL sender. If this field is null, it indicates that the client is connected via a Unix socket on the server machine.
client_hostname	text	Host name of the connected client, as reported by a reverse DNS lookup of client_addr. This field will only be non-null for IP connections, and only when log_hostname is enabled.
client_port	integer	TCP port number that the client is using for communication with this WAL sender, or -1 if a Unix socket is used
backend_start	timestamp	Time when this process was started, i.e., when the client connected to this WAL sender
state	text	Current WAL sender state
sent_location	text	Last transaction log position sent on this connection
write_location	text	Last transaction log position written to disk by this standby server
flush_location	text	Last transaction log position flushed to disk by this standby server
replay_location	text	Last transaction log position replayed into the database on this standby server
sync_priority	integer	Priority of this standby server for being chosen as the synchronous Standby
sync_state	text	Synchronous state of this standby server.

Select appropriate columns to view the replication status:

select client_addr, state, sync_priority, sent_location, write_location, flush_location, replay_location from pg_stat_replication;  

Also, another function would help out in Replication lag behind Stand by and Primary server. 

pg_last_xact_replay_timestamp() 

Using Current time and with above function, we get replication delay as

Kamal=# select now() - pg_last_xact_replay_timestamp() AS replication_delay_time;
 replication_delay_time
-----------------------
 00:00:04.064454
(1 row) 

Parellely, monitor the database size if this replication designed for multiple database.

 SELECT D.DATNAME AS NAME,  PG_CATALOG.PG_GET_USERBYID(D.DATDBA) AS OWNER,
    CASE WHEN PG_CATALOG.HAS_DATABASE_PRIVILEGE(D.DATNAME, 'CONNECT')
        THEN PG_CATALOG.PG_SIZE_PRETTY(PG_CATALOG.PG_DATABASE_SIZE(D.DATNAME))
        ELSE 'NO ACCESS'
    END AS SIZE
FROM PG_CATALOG.PG_DATABASE D
    ORDER BY
    CASE WHEN PG_CATALOG.HAS_DATABASE_PRIVILEGE(D.DATNAME, 'CONNECT')
        THEN PG_CATALOG.PG_DATABASE_SIZE(D.DATNAME)
        ELSE NULL
    END DESC
    LIMIT --;