Re: Fast AT ADD COLUMN with DEFAULTs

On 10/6/16, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Serge Rielau <serge(at)rielau(dot)com> writes:
>>> On Oct 6, 2016, at 5:25 AM, Vitaly Burovoy <vitaly(dot)burovoy(at)gmail(dot)com>
>>> wrote:
>>>> Which makes me think we should call this missing_value or absent_value

Be honest Simon Rigg's wrote that words.

>>>> 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.
>
>> I like Tom’s “creation default”. Another one could be “initial default”.
>> But that, too, can be misread.
>
> Something based on missing_value/absent_value could work for me too.
>
> If we name it something involving "default", that definitely increases
> the possibility for confusion with the regular user-settable default.
>
> Also worth thinking about here is that the regular default expression
> affects what will be put into future inserted rows, whereas this thing
> affects the interpretation of past rows. So it's really quite a different
> animal. That's kind of leading me away from calling it creation_default.
>
> BTW, it also occurs to me that there are going to be good implementation
> reasons for restricting it to be a hard constant, not any sort of
> expression. We are likely to need to be able to insert the value in
> low-level code where general expression evaluation is impractical.

Yes, I mentioned that it should be evaluated and stored as a value
because user functions can be changed (besides the speed reason),
that's why I like the "value" in its name. The "default" is usually
identified with expressions, not values (which are particular cases of
expressions).

Serge mentioned the phrase "pre-existing rows", which makes me think
about something like "pre_existing_value"....

--
Best regards,
Vitaly Burovoy