From here:
https://www.postgresql.org/about/featurematrix/detail/392/
UNIQUE NULLS NOT DISTINCT
By default, NULL values are not treated as distinct entries. Specifying
NULLS NOT DISTINCT on unique indexes / constraints will cause NULL
values to be treated distinctly.
From here:
https://www.postgresql.org/docs/current/sql-createindex.html
NULLS DISTINCT
NULLS NOT DISTINCT
Specifies whether for a unique index, null values should be
considered distinct (not equal). The default is that they are distinct,
so that a unique index could contain multiple null values in a column.
and here:
https://www.postgresql.org/docs/current/sql-createtable.html
For the purpose of a unique constraint, null values are not considered
equal, unless NULLS NOT DISTINCT is specified.
--
Adrian Klaver
adrian.klaver@aklaver.com