https://edu.postgrespro.ru/sqlprimer/sqlprimer-2019-msu-06.pdf
    ! p18

create unique index <index-name> on <table-name> (col1, col2, ...);
    ! nulls are accepted at unique indices

2024
https://habr.com/ru/articles/871546/
    ! each field with unique-constraint gets unique idx auto-created !!!