Need help requiring uniqueness in text columns

I have a table MESSAGE with a text column and a timestamp column. I
want to make sure that I have no rows with duplicates for both values.
I have two ideas right now for how to do this.

IDEA 1:

CREATE UNIQUE INDEX nodups on MESSAGE (my_timestamp_col, my_text_col);

IDEA 2:

CREATE UNIQUE INDEX nodups on MESSAGE (my_timestamp_col, md5(my_text_col));

I am speculating that an index on the md5 is cheaper than on a text
column. I'm willing to risk the chance of a hash collision.

I don't want to use this index to allow searching inside the text
column. I just want to protect against duplication.

Are either of these approaches any good? What are other ways to
guarantee uniqueness for the pair of my timestamp column and my text
column?

TIA

Matt

--
Programming, economics, gardening, life in Cleveland.
http://blog.tplus1.com