The Unique Indexes section (
https://www.postgresql.org/docs/15/indexes-unique.html) does not mention
the new NULLS [ NOT ] DISTINCT capability of indexes (
https://www.postgresql.org/docs/15/sql-createindex.html) and it probably
should.
Specifically, it has the exact wording from previous versions (emphasis
added):
When an index is declared unique, multiple table
rows with equal indexed values are not allowed.
*Null values are not considered equal*.
We should consider adding "unless the NULLS NOT DISTINCT clause is used
when creating the index", or something to that effect.