On Wed, Jul 27, 2016 at 7:35 AM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> "David G. Johnston" <david.g.johns...@gmail.com> writes:
>> The concept embodied by "NULL" in the operator "IS [NOT] NULL" is distinct
>> from the concept embodied by "NULL" in the operator "IS [NOT] DISTINCT
>> FROM".
>> In short, the former smooths out the differences between composite and
>> non-composite types while the later maintains their differences.  While a
>> bit confusing I don't see that there is much to be done about it - aside
>> from making the distinction more clear at:
>> https://www.postgresql.org/docs/devel/static/functions-comparison.html
>> Does spec support or refute this distinction in treatment?
> AFAICS, the IS [NOT] DISTINCT FROM operator indeed is specified to do the
> "obvious" thing when one operand is NULL: you get a simple nullness check
> on the other operand.  So I went ahead and documented that it could be
> used for that purpose.

By the way, our documentation says that NOT NULL constraints are
equivalent to CHECK (column_name IS NOT NULL).  That is what the SQL
standard says, but in fact our NOT NULL constraints are equivalent to
CHECK (column_name IS DISTINCT FROM NULL).  Should we update the
documentation with something like the attached?

Thomas Munro

Attachment: not-null-does-not-mean-check-is-not-null.patch
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