>>>>> "Tom" == Tom Lane <t...@sss.pgh.pa.us> writes:
>> 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;
Tom> While I agree that the UPDATE part of that desperately needs
Tom> improvement, I don't agree that the INSERT part is entirely fine.
Tom> You're still relying on a parse-time expansion of the (n).*
Tom> notation, which is inefficient
Not in my experience a huge deal given what else is going on...
Tom> and not at all robust against schema changes (the same problem as
Tom> with the patch's approach to UPDATE).
Now this I think is wrong; I think it's just as robust against schema
changes as using the composite value directly would be. Consider the
case where foo and reference_foo match with the exception of dropped
columns; the code as it is should just work, while a variant that used
the composite values would have to explicitly deal with that.
(When I've used this kind of operation in practice, reference_foo has
just been created using CREATE TEMP TABLE reference_foo (LIKE foo); and
then populated via COPY from an external data source. Even if
reference_foo were a non-temp table, the logic of the situation requires
it to have the same schema as foo as far as SQL statements are
concerned.)
Tom> So if we're taking this as a motivating example, I'd want to see a
Tom> fix that allows both INSERT and UPDATE directly from a composite
Tom> value of proper rowtype, without any expansion to individual
Tom> columns at all.
I would argue that this is a case of the perfect being the enemy of the
good.
Tom> Perhaps we could adopt some syntax like
Tom> INSERT INTO table (*) values-or-select
Tom> to represent the case that the values-or-select delivers a single
Tom> composite column of the appropriate type.
We could, but I think in all practical cases it'll be nothing more than
a small performance optimization rather than something that really
benefits people in terms of enhanced functionality.
>> 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.
Tom> Sure, but the patch as given didn't work very well for that
Tom> either,
Partly that's a limitation resulting from how much can be done with the
existing SET (...) = syntax and implementation without ripping it all
out and starting over. An incremental improvement seemed to be a more
readily achievable goal.
In practice it would indeed probably look like:
declare
new_id integer;
new_values hstore;
begin
/* do stuff */
update foo
set (*) = (select * from populate_record(foo, new_values))
where foo.id = new_id;
A agree that it would be nicer to do
update foo
set (*) = populate_record(foo, new_values)
where foo.id = new_id;
but that would be a substantially larger project. The alternative of
set * = populate_record(foo, new_values)
is less satisfactory since it introduces inconsistencies with the case
where you _do_ want to specify explicit columns, unless you also allow
set (a,b) = row_value
which is required by the spec for T641 but which we don't currently
have.
--
Andrew (irc:RhodiumToad)
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