Re: Support UPDATE table SET(*)=...

Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> writes:
> "Tom" == Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
> Tom> I spent a fair amount of time cleaning this patch up to get it
> Tom> into committable shape, but as I was working on the documentation
> Tom> I started to lose enthusiasm for it, because I was having a hard
> Tom> time coming up with compelling examples.

> One example that comes up occasionally (and that I've had to do myself
> more than once) is this: given a table "foo" and another with identical
> schema "reference_foo", apply appropriate inserts, updates and deletes
> to table "foo" to make the content of the two tables identical. This can
> be done these days with wCTEs:

> with
> t_diff as (select o.id as o_id, n.id as n_id, o, n
> from foo o full outer join reference_foo n on (o.id=n.id)
> where (o.*) is distinct from (n.*)),
> ins as (insert into foo select (n).* from t_diff where o_id is null),
> del as (delete from foo
> where id in (select o_id from t_diff where n_id is null)),
> upd as (update foo
> set (col1,col2,...) = ((n).col1,(n).col2,...) -- XXX
> from t_diff
> where foo.id = n_id and o_id = n_id)
> select count(*) filter (where o_id is null) as num_ins,
> count(*) filter (where o_id = n_id) as num_upd,
> count(*) filter (where n_id is null) as num_del
> from t_diff;

While I agree that the UPDATE part of that desperately needs improvement,
I don't agree that the INSERT part is entirely fine. You're still relying
on a parse-time expansion of the (n).* notation, which is inefficient and
not at all robust against schema changes (the same problem as with the
patch's approach to UPDATE). So if we're taking this as a motivating
example, I'd want to see a fix that allows both INSERT and UPDATE directly
from a composite value of proper rowtype, without any expansion to
individual columns at all.

Perhaps we could adopt some syntax like
INSERT INTO table (*) values-or-select
to represent the case that the values-or-select delivers a single
composite column of the appropriate type.

> Other examples arise from things one might want to do in plpgsql; for
> example to update a record from an hstore or json value, one can use
> [json_]populate_record to construct a record variable, but then it's
> back to naming all the columns in order to actually perform the update
> statement.

Sure, but the patch as given didn't work very well for that either,
at least not if you wanted to avoid multiple evaluation of the
composite-returning function. You'd have to adopt some obscure syntax
like "UPDATE target SET (*) = (SELECT * FROM composite_function(...))".
With what I'm thinking about now you could do
UPDATE target SET * = composite_function(...)
which is a good deal less notation, and with a bit of luck it would not
require disassembling and reassembling the function's output tuple.

regards, tom lane