How to check the redo apply gap on an Oracle Data Guard standby

Two views of the same question: how far behind is the standby? Query 1 is single-instance and compares max sequence received (archived=''YES'') against max sequence applied (applied=''YES'') per thread. Query 2 uses GV$ views and adds host, instance, DB name, and the timestamp of the last applied archive — useful on a RAC standby.

-- 1) Simple gap (single-instance), tied to the current incarnation
SET LINES 800 PAGESIZE 10000
BREAK ON REPORT
COMPUTE SUM LABEL TOTAL OF gap ON REPORT

SELECT primary.thread#,
       primary.maxsequence AS primary_seq,
       standby.maxsequence AS standby_seq,
       primary.maxsequence - standby.maxsequence AS gap
FROM (SELECT thread#, MAX(sequence#) AS maxsequence
      FROM   v$archived_log
      WHERE  archived           = 'YES'
      AND    resetlogs_change#  = (SELECT resetlogs_change# FROM v$database)
      GROUP BY thread#) primary,
     (SELECT thread#, MAX(sequence#) AS maxsequence
      FROM   v$archived_log
      WHERE  applied            = 'YES'
      AND    resetlogs_change#  = (SELECT resetlogs_change# FROM v$database)
      GROUP BY thread#) standby
WHERE  primary.thread# = standby.thread#;


-- 2) RAC-aware gap with host, instance, DB name, and last-applied timestamp
SET LINES 1000
COL host_name          FORMAT a32
COL last_apply         FORMAT a20
COL db_unique_name     FORMAT a15
COL db_name            FORMAT a10

SELECT DISTINCT
       i.host_name,
       i.instance_name,
       a.thread#,
       d.name          AS db_name,
       d.db_unique_name,
       b.last_seq,
       a.applied_seq,
       TO_CHAR(a.last_app_timestamp, 'DD/MM/YYYY HH24:MI:SS') AS last_apply,
       b.last_seq - a.applied_seq AS arc_diff
FROM (SELECT thread#,
             MAX(sequence#) AS applied_seq,
             MAX(next_time) AS last_app_timestamp
      FROM   gv$archived_log
      WHERE  applied IN ('YES', 'IN-MEMORY')
      GROUP BY thread#) a,
     (SELECT thread#, MAX(sequence#) AS last_seq
      FROM   gv$archived_log
      GROUP BY thread#) b,
     gv$instance i,
     gv$database d
WHERE  a.thread# = b.thread#
AND    a.thread# = i.thread#
AND    i.con_id  = d.con_id;