Composite types questions

Hi all.

It's not clear to me how composite values are used in  conditions (WHERE/CHECK).
In my case I have something like this:

-- begin snippet

CREATE TABLE test_tab (
  col1 timestamp not null,
  col2 int8 not null,
  col3 text not null
);

CREATE INDEX i_test_tab_col1 ON test_tab( col1 );

SELECT *
  FROM test_tab
  WHERE (date_trunc('week',col1),col2,col3)=('val1'::timestamp,val2,'val3')
;
-- end snippet

For a number of reasons I cannot split the the WHERE condition in the
"usual" AND-list.
My questions are:
Can the i_test_tab_col1 INDEX be taken into account from the query planner?
What if I define a functional index on col1?
Does the same apply to TABLE-level CHECK conditions?

Thanks in advance.

--
Vincenzo Romano
NotOrAnd Information Technologies
NON QVIETIS MARIBVS NAVTA PERITVS