Обсуждение: Documentation discrepancy UNIQUE NULLS NOT DISTINCT
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
On Thu, Jan 19, 2023 at 3:41 PM Adrian Klaver <adrian.klaver@aklaver.com> wrote:
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.
I think saying that NULLs are distinct by default is the better way to present this. The documentation disagrees on the verbiage but both state the same truth; the feature matrix page needs to be fixed.
David J.