Re: Add support for data change delta tables

On 1/13/24 12:41, PavelTurk wrote:
> Hello all,

Hi Pavel!

> Currently PostgreSQL doesn't support data change delta tables. For
> example, it doesn't support this type of query:
>
> SELECT * FROM NEW TABLE (
>     INSERT INTO phone_book
>     VALUES ( 'Peter Doe', '555-2323' )
> ) AS t

Correct. We do not yet support that.

> PostgreSQL has RETURNING that provides only a subset of this functionality.

I think that because of the way postgres is designed, it will only ever
provide a subset of that functionality anyway. Other people know more
of the internals than I do, but I don't think we can easily distinguish
between NEW TABLE and FINAL TABLE.

Unfortunately, your example does not show how postgres is inadequate.

For example,

INSERT INTO t1 (c1)
SELECT c2
FROM OLD TABLE (
DELETE FROM t2
WHERE ...
) AS t

can be written as

WITH
old_table (c2) AS (
DELETE FROM t2
WHERE ...
RETURNING c2
)
INSERT INTO t1 (c1) TABLE old_table

> So I suggest to add support for data change delta tables. Because this
> feature is more powerful and it is included
> in the SQL Standard.

It is indeed included in the SQL Standard, but is it really more powerful?

Consider this example which is currently not implemented but could be,
and compare it to the standard where such a query could not be possible
at all:

UPDATE t
SET a = ...
WHERE ...
RETURNING OLD.a, NEW.a, FINAL.a

All this being said, I would love to see data change delta tables
implemented per-spec in PostgreSQL; in addition to improving the
RETURNING clause. I believe I have heard that we can't just do a
syntactic transformation because the trigger execution order is not the
same, but I would have to research that.
--
Vik Fearing