How to catch a particular exception?

It is possible to catch an exception in plpgsql: EXCEPTION WHEN unique_violation THEN But how to check for a violation of a particular constraint? The following example creates two keys: one for a and one for b. How to know which one has caused the exception? ~~~sql create table t ( a text unique, b text unique ); create procedure insert_t(a text, b text) language plpgsql as $$ begin insert into t values (a, b); exception when unique_violation then raise notice 'is it a or b?'; end $$; call insert_t('x', 'foo'); call insert_t('x', 'bar'); select * from t; ~~~