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 http://www.enterprisedb.com
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