Re: To-Do item: skip table scan for adding column with provable check constraints

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:

> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> Right. If there were a DEFAULT on the new column that would of course
>> be different, and you can also do thinks like CHECK (a != b) here.
>> However, if the CHECK constraint does not reference any column other
>> than the newly-added one, and if the new column will have the same
>> value for every row either because there is no default or because the
>> default is a constant,
>
> ... and if the CHECK expression is immutable ...

Doesn't it have to be already? Otherwise a value accepted at one point
in time could suddenly violate the constraint later, even though it
never changed.

ilmari

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