What is Oracle's v$session_longops equivalent in Postgres?

You can find long running queries in postgres by running a script like belor:

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

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where you can change the << '5 >> minutes interval to whatever you wish like << "10  >> for 10seconds etc...

https://www.shanelynn.ie/postgresql-find-slow-long-running-and-blocked-queries/

The query below should give a list with all executing/running queries including their runtime:

SELECT pid, age(clock_timestamp(), query_start), datname, usename, query, state
FROM pg_stat_activity
WHERE state NOT LIKE 'idle%' AND query NOT ILIKE '%pg_stat_activity%'
ORDER BY query_start desc;

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That query will show you how long a query has been running.

I'm not sure Postgres has the other information contained in v$session_longops.  Definitely have never seen anything like TIME_REMAINING in Postgres.

Some of the other columns might be obtained from other queries.