Re: [HACKERS] identity columns - Mailing list pgsql-hackers

On 3/22/17, Peter Eisentraut <peter.eisentraut@2ndquadrant.com> wrote:
> On 3/22/17 03:59, Vitaly Burovoy wrote:
>> Column's IDENTITY behavior is very similar to a DEFAULT one. We write
>> "SET DEFAULT" and don't care whether it was set before or not, because
>> we can't have many of them for a single column. Why should we do that
>> for IDENTITY?
>
> One indication is that the SQL standard requires that DROP IDENTITY only
> succeed if the column is currently an identity column.  That is
> different from how DEFAULT works.

I think we'll end up with "DROP IDENTITY IF EXISTS" to avoid raising
an exception and "ADD OR SET" if your grammar remains.

> Another difference is that there is no such thing as "no default",
> because in absence of an explicit default, it is NULL.  So all you are
> doing with SET DEFAULT or DROP DEFAULT is changing the default.  You are
> not actually adding or removing it.

Right. From that PoV IDENTITY also changes a default value: "SET (ADD
... AS?) IDENTITY" works as setting a default to "nextval(...)"
whereas "DROP IDENTITY" works as setting it back to NULL.

> Therefore, the final effect of SET DEFAULT is the same no matter whether
> another default was there before or not.  For ADD/SET IDENTITY, you get
> different behaviors.  For example:
>
> ADD .. AS IDENTITY (START 2)
>
> creates a new sequence that starts at 2 and uses default parameters
> otherwise.  But
>
> SET (START 2)
>
> alters the start parameter of an existing sequence.  So depending on
> whether you already have an identity sequence, these commands do
> completely different things.

If you use "SET START 2" to a non-identity columns, you should get an
exception (no information about an identity type: neither "by default"
nor "always"). The correct example is:
ADD GENERATED BY DEFAULT AS IDENTITY (START 2)
and
SET GENERATED BY DEFAULT SET START 2

Note that creating a sequence is an internal machinery hidden from users.
Try to see from user's PoV: the goal is to have a column with an
autoincrement. If it is already autoincremented, no reason to create a
sequence (it is already present) and no reason to restart with 2
(there can be rows with such numbers).
"... SET START 2" is for the next "RESTART" DDL, and if a user insists
to start with 2, it is still possible:

SET GENERATED BY DEFAULT SET START 2 RESTART 2


I still think that introducing "ADD" for a property which can not be
used more than once (compare with "ADD CHECK": you can use it with the
same expression multiple times) is not a good idea.

I think there should be a consensus in the community for a grammar.

> --
> Peter Eisentraut              http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

-- 
Best regards,
Vitaly Burovoy