Numeric with scale precision always shows the trailing zeros.
Numeric plain only shows numbers after the decimal point that are being
used.


That statement is false:

regression=# select 1234.000::numeric;
 numeric  
----------
 1234.000
(1 row)

I'm not sure offhand what is the easiest way to suppress trailing
zeroes, but casting to plain numeric is not the route to a solution.

Really this is a textual formatting problem.  You might find that the
best bet is something with trim() or a regexp.  The trick would be
not removing zeroes that are significant ...

			regards, tom lane

According to the documentation, numeric is stored without any leading or trailing zeros.
http://www.postgresql.org/docs/current/static/datatype-numeric.html

Numeric values are physically stored without any extra leading or trailing zeroes. Thus, the declared precision and scale of a column are maximums, not fixed allocations. (In this sense the numeric type is more akin to varchar(n) than to char(n).) The actual storage requirement is two bytes for each group of four decimal digits, plus eight bytes overhead.

However, in practice:
create table test(f1 numeric);
insert into test(f1)values(15.000);
select * from test;
f1
-------
15.000