Re: Document NULL

On Thu, 2024-05-02 at 08:23 -0700, David G. Johnston wrote:
> Version 2 attached.  Still a draft, focused on topic picking and overall structure.

I'm fine with most of the material (ignoring ellipses and typos), except this:

+    The NOT NULL column constraint is largely syntax sugar for the corresponding
+    column IS NOT NULL check constraint, though there are metadata differences
+    described in create table.

I see a substantial difference there:

  SELECT conname, contype,
         pg_get_expr(conbin, 'not_null'::regclass)
  FROM pg_constraint
  WHERE conrelid = 'not_null'::regclass;

         conname        │ contype │   pg_get_expr
  ══════════════════════╪═════════╪══════════════════
   check_null           │ c       │ (id IS NOT NULL)
   not_null_id_not_null │ n       │ ∅
  (2 rows)

There is also the "attnotnull" column in "pg_attribute".

I didn't try it, but I guess that the performance difference will be measurable.
So I wouldn't call it "syntactic sugar".

Perhaps: The behavior of the NOT NULL constraint is like that of a check
constraint with IS NOT NULL.

Yours,
Laurenz Albe