Quick flyby here...
On Tuesday, May 3, 2016, Tom Lane <t...@sss.pgh.pa.us> wrote:
> >> Point 2 is where things differ from what I remember; my (possibly
> >> flawed) understanding was that there's no difference between those
> >> things. Many (maybe all) of the things from this point on are probably
> >> fallout from that one change.
> > It is just mentioning that CHECK constraints have influence on
> > nullability characteristic, but it differs from NNC.
> > NNC creates CHECK constraint, but not vice versa. You can create
> > several CHECK "col IS NOT NULL" constraints, but only one NNC (several
> > ones by inheritance only?). And DROP NOT NULL should drop only those
> > CHECK that is linked with NNC (and inherited), but no more (full
> > explanation is in my initial letter).
Either it's one, or it's not...
> This seems to me to be a most curious reading of the standard.
> SQL:2011 11.4 <column definition> syntax rule 17a says
> If a <column constraint definition> is specified that contains
> the <column constraint> NOT NULL, then it is equivalent to the
> following <table constraint definition>:
> CND CHECK ( C IS NOT NULL ) CA
> As a rule, when the SQL spec says "equivalent", they do not mean "it's
> sort of like this", they mean the effects are indistinguishable. In
> particular, I see nothing whatsoever saying that you're not allowed to
> write more than one per column.
Does it define how DROP NOT NULL is supposed to behave?
I agree that the behavior of a column NNC is identical to a similar
constraint defined on the table: but if drop not null doesn't impact table
constraints then the concept of perfect equality is already lost.
> So I don't like the proposal to add an attnotnullid column to
> pg_attribute. What we'd talked about earlier was converting attnotnull
> into, effectively, a hint flag saying that there's at least one NOT NULL
> constraint attached to the column.
Have we considered making it a table constraint and giving it a name? We
already handle that case without difficulty.
Not looking for a detailed explanation.