Find difference between timestamps in seconds in PostgreSQL

Try: 

SELECT EXTRACT(EPOCH FROM (timestamp_B - timestamp_A))
FROM TableA

Details here: EXTRACT.

select age(timestamp_A, timestamp_B)

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Answering to Igor's comment:

select age('2013-02-28 11:01:28'::timestamp, '2011-12-31 11:00'::timestamp);
              age              
-------------------------------
 1 year 1 mon 28 days 00:01:28