Q:

Alter table decimal field?

I tried to alter table with decimal type:
porog   decimal(6,0)

When I change range on the (6, 4) and save it, I get again (6,0). Why?
Now I have field decimal(15,14)
When I insert value:
3,41557740625523
I get: 3.00000000000000

A:

I built up a simple scheme to test the things you mentioned, but I cannot reproduce your problems concerning the loss of after-comma digits. Can it be that any sql client or query does some rounding? Anyway, have a look at the scheme, the inserts and the output. Maybe it helps and answers at least your question about "What decimal format I should use?":
create table test (
  yourfield decimal(15,14),
  onlyaftercomma decimal(18,18)
);
insert into test (yourfield,onlyaftercomma) values
  (3.41557740625523, 0.00041557740625523);

Then...
select * from test

yields...
yourfield        | onlyaftercomma
3.41557740625523 | 0.00041557740625523

