On Tue, Jul 5, 2011 at 12:42 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Sun, Jul 3, 2011 at 12:31 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> The code that recognizes a default expression as being just constant
>>> NULL doesn't think this is a constant NULL. In principle it could
>>> recognize that, since the cast function is marked strict, but so far
>>> it has not seemed worth the trouble.
>> Gee, does Noah's recent patch adding the notion of "transform
>> functions" have any applicability to this problem?
> Not really. If someone held a gun to my head and said "fix that", what
> I'd do is run eval_const_expressions() on the default expression and see
> if that resulted in a constant NULL. But it seems unlikely to be worth
> the cycles in most cases. Also, we'd then need some other test to
> address the issue explained in AddRelationNewConstraints:
> * If the expression is just a NULL constant, we do not bother to make
> * an explicit pg_attrdef entry, since the default behavior is
> * equivalent.
> * Note a nonobvious property of this test: if the column is of a
> * domain type, what we'll get is not a bare null Const but a
> * CoerceToDomain expr, so we will not discard the default. This is
> * critical because the column default needs to be retained to
> * override any default that the domain might have.
> if (expr == NULL ||
> (IsA(expr, Const) &&((Const *) expr)->constisnull))
> IOW, there are cases where "DEFAULT NULL" is *not* a no-op.
Interesting. A possible reason to care about this is that it might
convert a form of ALTER TABLE that requires a rewrite into one that
doesn't, since we needn't rewrite the table if the column will be
all-nulls. That's not enough of a benefit to motivate me to do the
work myself, since all the examples thus-far shown involve writing the
default in a way that's more complicated than necessary. But I'd have
a hard time objecting if someone else wanted to run it down, since I'm
pretty sure I've written an ALTER TABLE that way once or twice myself.
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