Constraint on multicolumn index

Hi,

I am not sure this can be done but I'm trying to constrain a sorted set
efficiently using a multicolumn index in postgres. The (simplified)
scenario is this:

CREATE TABLE T
(
a INT,
b INT,
c INT
);

CREATE INDEX t_idx ON T(a,b,c);

Now I can sort using t_idx:

select * from T order by a,b,c; -- all good, seq scan using t_idx

I can constrain on a single variable fine:

select * from T where (a=10 AND b=100 AND c>1000) order by a,b,c;
-- does seq scan on t_idx and uses the index in the constraint as
expected

But if I want the next item following t=(a=10,b=100,c=1000):

select * from T
where (a=10 AND b=100 AND c>1000) OR (a=10 AND b>100) OR (a>10)
order by a,b,c;

then it just does an ordinary filter, and basically does a sequence scan
with no intelligence which isn't great if you've got a table of 20
million items.

Is there any way short of issuing 3 queries and joining them that I can
do this? I had hoped to be able to compare (a,b,c)>(10,100,1000) but of
course that evaluates to (a>10) and (b>100) and (c>1000). It feels like
there should be a simple solution to this... please help :)

Thanks
Stuart