Thanks for your reply. I read it.
When I restart secondary node, it reported "DEBUG:  recovery snapshot waiting for non-overflowed snapshot or until oldest active xid on standby is at least 4739126 (now 1422751)#0122020-12-21 00:00:24.415 UTC [146-15455940] CONTEXT:  WAL redo at 4/741941F8 for Standby/RUNNING_XACTS: nextXid 5137417 latestCompletedXid 5137409 oldestRunningXid 1422751; 17 xacts: 2716862 2721890 4665244 2495592 2289138 5137416 2288820 2287653 1422751 4280517 2288510 2287620 3297674 1757103 5137219 3320989 2259670; subxid ovf"
I want to find the oldest running transaction(1422751) using the following SQL. But it showed that there was no transactions that running more than 5 minutes. 
How can I find the oldest running transaction?

SELECT
  pid,
  now() - pg_stat_activity.query_start AS duration,
  query,
  state
FROM pg_stat_activity
WHERE (now() - pg_stat_activity.query_start) > interval '5 minutes';