On 3/21/17, Peter Eisentraut <peter.eisentr...@2ndquadrant.com> wrote:
> On 3/21/17 16:11, Vitaly Burovoy wrote:
>> My argument is consistency.
>> Since IDENTITY is a property of a column (similar to DEFAULT, NOT
>> NULL, attributes, STORAGE, etc.), it follows a different rule: it is
>> either set or not set. If it did not set before, the "SET" DDL "adds"
>> it, if that property already present, the DDL replaces it.
>> There is no "ADD" clause in DDLs like "...ALTER table ALTER column..."
>> (only "SET", "RESET" and "DROP")[2].
>> Your patch introduces the single DDL version with "...ALTER column
>> ADD..." for a property.
> But it creates a sequence, so it creates state.

Right. But it is an internal mechanism. DDL is not about creating a
sequence, it is about changing a property.

> So mistakes could easily be masked.  With my patch, if you do ADD twice, you 
> get an error.

Agree. But what's for? Whether that parameters are incompatible (and
can't be changed later)?

> With your proposal, you'd have to use SET, and you could overwrite
> existing sequence state without realizing it.

I can't overwrite its state (current value), only its settings like
start, maxval, etc.

In fact when I write a DDL I want to change a schema. For
non-properties it is natural to write "CREATE" (schema, table) and
"ADD" (column, constraints) because there can be many of them (with
different names) in a single object: many schemas in a DB, many tables
in a schema, many columns in a table and even many constraints in a
table. So ADD is used for adding objects which have a name to some
container (DB, schema, table).
It is not true for the IDENTITY property. You can have many identity
columns, but you can not have many of them in a single column.

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

Whether I write "ADD" or "SET" I want to have a column with some
behavior and I don't mind what behavior it has until it is
incompatible with my wish (e.g. it has DEFAULT, but I want IDENTITY or
vice versa).

>>> It does change the type, but changing the type doesn't change the
>>> limits.  That is a property of how ALTER SEQUENCE works, which was
>>> separately discussed.
>> Are you about the thread[1]? If so, I'd say the current behavior is not
>> good.
>> I sent an example with users' bad experience who will know nothing
>> about sequences (because they'll deal with identity columns).
>> Would it be better to change bounds of a sequence if they match the
>> bounds of an old type (to the bounds of a new type)?
> That's an idea, but that's for a separate patch.

It is very likely to have one in Postgres10. I'm afraid in the other
case we'll impact with many bug reports similar to my example.

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

Best regards,
Vitaly Burovoy

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