You have two timestamp values:
timestamp2 and want to find the difference between the two.
If you want the results in hours, months, days, hours, etc:
SELECT age(timestamp1, timestamp2);
If you want the results in seconds only:
SELECT EXTRACT(EPOCH FROM timestamp 'timestamp1') - EXTRACT(EPOCH FROM timestamp 'timestamp2');
Or casting it this way:
SELECT EXTRACT(EPOCH FROM ('timestamp1'::timestamp - 'timestamp2'::timestamp));
SELECT EXTRACT(EPOCH FROM ('2018-06-28 12:01:38'::timestamp - '2017-12-11 11:30'::timestamp));
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