On 10/5/16, Andres Freund <andres@anarazel.de> wrote:
> On 2016-10-05 15:23:05 -0700, Vitaly Burovoy wrote:
>> On 10/5/16, Andres Freund <andres@anarazel.de> wrote:
>> > On 2016-10-05 11:58:33 -0700, Serge Rielau wrote:
>> >> Dear Hackers,
>> >> I’m working on a patch that expands PG’s ability to add columns to a
>> >> table
>> >> without a table rewrite (i.e. at O(1) cost) from the
>> >> nullable-without-default to a more general case.
>> >
>> > If I understand this proposal correctly, altering a column default will
>> > still have trigger a rewrite unless there's previous default?
>>
>> No, "a second “exist default"" was mentioned, i.e. it is an additional
>> column in a system table (pg_attribute) as default column values of
>> the "pre-alter" era. It solves changing of the default expression of
>> the same column later.
>
> Don't think that actually solves the issue. The default might be unset
> for a while, for example. Essentially you'd need to be able to associate
> arbitrary number of default values with an arbitrary set of rows.
>
> ALTER TABLE foo ALTER COLUMN withdefault DROP DEFAULT;
> INSERT id = 1;
> ALTER TABLE foo ALTER COLUMN withdefault SET DEFAULT 1;
> ALTER TABLE foo ALTER COLUMN withdefault DROP DEFAULT;
> INSERT id = 2;
> ALTER TABLE foo ALTER COLUMN withdefault SET DEFAULT 2;
> ALTER TABLE foo ALTER COLUMN withdefault DROP DEFAULT;
> INSERT id = 3;
> ALTER TABLE foo ALTER COLUMN withdefault SET DEFAULT 3;
>
> The result here would be that there's three rows with a default value
> for foo that's the same as their id. None of them has that column
> present in the row.
I'm sorry, while I was writting "pre-alter" I meant
"pre-alter-add-column" era (not "pre-alter-set-default"), all later
default changes "current" default, whereas "pre-alter-add-column" adds
value if current column number < TupleDesc.natts.
All your DDL are in the "post-alter-add-column" era.
--
Best regards,
Vitaly Burovoy
