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.kla...@aklaver.com


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