tricky CHECK condition

Multi-column unique index?

How could I specify a condition in 'CHECK' constraint for a column to
check that its value must only appear once in *some* rows of a table?
Sort of a local "UNIQUE"... For example, in CDs world:

create table tracks (
-- Unique track identifier
id serial primary key,
-- Which disk the track is from
disk_id int4 references disks (id),
-- Track number in a disk
track_number int2,
check (WHAT I WANT GOES HERE) );

I want to check values of `track_number' to be unique for the
disk. Basically, I want to check that an SQL query

select track_number from tracks T
where T.disk_id=disk_id and T.track_number=track_number;

is empty... Is it possible to run an SQL query in a condition clause?
I looked thru documentation, but couldn't find anything relevant.
--
Arcady Genkin http://www.thpoon.com
Nostalgia isn't what it used to be.

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  pgman@candle.pha.pa.us               |  (610) 853-3000
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Bruce Momjian <pgman@candle.pha.pa.us> writes:

Multi-column unique index?

Excellent! Thank you very much!
--
Arcady Genkin
Nostalgia isn't what it used to be.

Arcady Genkin <a.genkin@utoronto.ca> writes:

I want to check values of `track_number' to be unique for the
disk.

Not sure if it can be done with a CHECK condition, but the traditional
solution is to create a unique index on the two columns:

create unique index tracks_disk_track_i on tracks(disk_id, track_number);

Updating an index should be a lot faster than running a whole subquery
for each insert, and the planner may be able to use the index to speed
up other queries too. So, it's a win all round...

regards, tom lane