Re: [HACKERS] identity columns

On 4/18/17, Peter Eisentraut <peter.eisentraut@2ndquadrant.com> wrote:
> On 4/7/17 01:26, Vitaly Burovoy wrote:
>> I've implement SET GENERATED ... IF NOT EXISTS. It must be placed
>> before other SET options but fortunately it conforms with the
>> standard.
>> Since that form always changes the sequence behind the column, I
>> decided to explicitly write "[NO] CACHE" in pg_dump.
>>
>> As a plus now it is possible to rename the sequence behind the column
>> by specifying SEQUENCE NAME in SET GENERATED.
>>
>> I hope it is still possible to get rid of the "ADD GENERATED" syntax.
>
> I am still not fond of this change.  There is precedent all over the
> place for having separate commands for creating a structure, changing a
> structure, and removing a structure.  I don't understand what the
> problem with that is.
>
> --
> Peter Eisentraut              http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


OK. Let's go through it again.
IDENTITY is a property of a column. There are no syntax to change any
property of any DB object via the "ADD" syntax.
Yes, a structure (a sequence) is created. But in fact it cannot be
independent from the column at all (I remind you that according to the
standard it should be unnamed sequence and there are really no way to
do something with it but via the column's DDL).
It is even hard to detect which sequence (since they have names) is
owned by the column:

postgres=# CREATE TABLE xxx(i int generated always as identity, j serial);
CREATE TABLE
postgres=# \d xxx*                           Table "public.xxx"Column |  Type   | Collation | Nullable |
Default
--------+---------+-----------+----------+--------------------------------i      | integer |           | not null |
generatedalways as identityj      | integer |           | not null | nextval('xxx_j_seq'::regclass)
 
Sequence "public.xxx_i_seq"  Column   |  Type   | Value
------------+---------+-------last_value | bigint  | 1log_cnt    | bigint  | 0is_called  | boolean | f
Sequence "public.xxx_j_seq"  Column   |  Type   | Value
------------+---------+-------last_value | bigint  | 1log_cnt    | bigint  | 0is_called  | boolean | f
Owned by: public.xxx.j


I can only guess that "public.xxx_i_seq" is owned by "public.xxx.i",
nothing proves that.
Whereas for regular sequence there are two evidences ("Default" and "Owned by").

Also the created sequence cannot be deleted (only with the column) or
left after the column is deleted.


Everywhere else the "ADD" syntax is used where you can add more than
one object to the altered one:
ALTER TABLE ... ADD COLUMN /* there can be many added columns
differing by names in the table */
ALTER TABLE ... ADD CONSTRAINT /* there can be many added constraints
differing by names in the table */
ALTER TYPE ... ADD VALUE /* many values in an enum differing by names
in the enum */
ALTER TYPE ... ADD ATTRIBUTE /* many attributes in a composite
differing by names in the enum */
etc.

But what is for "ALTER TABLE ... ALTER COLUMN ... ADD GENERATED"?
Whether a property's name is used for a distinction between them in a column?
Whether it is possible to have more than one such property to the
altering column?


The "SET GENERATED" (without "IF NOT EXISTS") syntax conforms to the
standard for those who want it.
The "SET GENERATED ... IF NOT EXISTS" syntax allows users to have the
column be in a required state (IDENTITY with set options) without
paying attention whether it is already set as IDENTITY or not.


The "[ NOT ] EXISTS" is a common Postgres' syntax extension for
creating/updating objects in many places. That's why I think it should
be used instead of introducing the new "ADD" syntax which contradicts
the users' current experience.


-- 
Best regards,
Vitaly Burovoy