Get seconds of day in Postgres

extract() the epoch from the time component after casting to time (effectively removing the date component):

SELECT extract(epoch FROM ts::time) AS seconds_of_day

You get the "number of seconds", including fractional seconds if there are any.
Very short and fast.

Test (with timestamps in unambiguous ISO format):

SELECT extract(epoch FROM ts::time) AS seconds_of_day
FROM  (
   VALUES 
     ('2014-10-12 00:00:30'::timestamp)   --    30
   , ('2014-10-12 00:01:30')              --    90
   , ('2014-10-12 00:02:00')              --   120
   , ('2014-10-12 12:00:00')              -- 43200
   , ('1999-12-23 23:59:59')              -- 86399
   , ('1999-12-23 23:59:59.123456')       -- 86399.123456
  ) t(ts);

db<>fiddle here

Use the extract() method:

select extract(second from current_timestamp) +
       extract(minute from current_timestamp) * 60 +
       extract(hour from current_timestamp) * 60 * 60;

of course this can be put into a function:

create or replace function total_seconds(p_timestamp timestamp)
 returns int
as
$$
  select (extract(second from p_timestamp) +
         extract(minute from p_timestamp) * 60 +
         extract(hour from p_timestamp) * 60 * 60)::int;
$$
language sql;

more details are in the manual:
http://www.postgresql.org/docs/current/static/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT