UNIQUE constraint on large VARCHARs - PostgreSQL

I have a column defined like

data_url character varying(32768) NOT NULL

and UNIQUE constraint on that column

CONSTRAINT unique_data_url UNIQUE (data_url)

When large object is being inserted into database, follow error message appears

ERROR: index row requires 32584 bytes, maximum size is 8191

How can be PostgreSQL set in order to be able to index object larger that 8191 chars?

Space and speed are not problem. It is a rarely changed table of at most hundreds of rows.

Environment: PostgreSQL 9.3.6, Fedora 20 x64