Possible Solution to Unique-With-NULL Problem

unique_with_null.sql

-- you may have to declare the function after the table exists.
CREATE FUNCTION check_unique_ex(TEXT, TEXT, TEXT)
RETURNS BOOLEAN
LANGUAGE SQL
AS $BODY$
  SELECT EXISTS (
    SELECT 1
    FROM ex
    WHERE a IS NOT DISTINCT FROM $1 AND
          b IS NOT DISTINCT FROM $2 AND
          C IS NOT DISTINCT FROM $3
  );
$BODY$;

CREATE TABLE ex (
  a TEXT,
  b TEXT,
  c TEXT,

  -- if you have to move the function declaration later,
  -- this will have to be moved to an ALTER TABLE
  CONSTRAINT unique_with_nulls CHECK (check_unique_ex(a, b, c)) 
);

-- these may or may not improve your performance.
-- not sure if a multi-column index would do better/worse
CREATE INDEX idx_a ON ex (a);
CREATE INDEX idx_b ON ex (b);
CREATE INDEX idx_c ON ex (c);