Select
   S.RDB$FIELD_NAME
from RDB$INDEX_SEGMENTS S
left join RDB$RELATION_CONSTRAINTS RC on (RC.RDB$INDEX_NAME = S.RDB$INDEX_NAME)
where
   RC.RDB$RELATION_NAME = 'PESSOA' -- Coloque aqui o nome da tabela
   and RC.RDB$CONSTRAINT_TYPE = 'PRIMARY KEY'