Re: Fast AT ADD COLUMN with DEFAULTs

On 10/6/16, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On 6 October 2016 at 04:43, Serge Rielau <serge(at)rielau(dot)com> wrote:
>>>> Or should I compose some sort of a design document?
>
> Having read this thread, I'm a little unclear as to what you're
> writing now, though there's definitely good ideas here.
>
> I think it would be beneficial to write up a single coherent
> description of this, including behaviour and a small sketch of
> implementation, just so everyone knows what this is. No design doc,
> but a summary.

At the moment I think it can also be a good idea to post the current
patch as a Proposal or a WIP to get initial feedback.

> It would be very useful to be able to do this...
> ALTER TABLE foo ADD last_updated_timestamp timestamp default
> current_timestamp
> so that it generates a constant value and stores that for all prior
> rows, but then generates a new value for future rows.

Yes, it works for stable "now()" but does not work for volatile
functions like "random()", "uuid_generate_v4()" or default for serial
columns. The only possible way I can see is to check an expression has
only "T_Const"s, static and stable functions. In such case the
expression can be evaluated and the result be saved as a value for
absented attributes of a tuple. In the other case save NULL there and
rewrite the table.

> Which makes me think we should call this missing_value or absent_value
> so its clear that it is not a "default" it is the value we use for
> rows that do not have any value stored for them.

It is definitely a default for a user, it is not a regular default internally.
I'm not a native speaker, "absent_value" can be mixed up with a NULL.
As for me the best phrase is "pre-add-column-default", but it is
impossible to use it as a column name. :-(
It is still an open question.

(I remember funny versions in a discussion[1] when people tried to
choose a name for a function reversed to pg_size_pretty...)

[1] https://www.postgresql.org/message-id/flat/CAFj8pRD-tGoDKnxdYgECzA4On01_uRqPrwF-8LdkSE-6bDHp0w(at)mail(dot)gmail(dot)com

--
Best regards,
Vitaly Burovoy