Thread: Updateable views
Hi,
I'm currently working with Bernd in an implementation
of updateable views and want to know the hacker's
opinion on this issue.
What features have to be implemented in a first
extension in order to the patch to be accepted? What
features can wait until a second extension?
This are my first thought on this (i start working on
this just two weeks ago).
***************** thoughts *******************
- What if we cannot create one of the three rules? Make the rule not updateable at all? Or create the rules we can?
(ithink this is the correct)
General Restrictions!!!
---------------------------
- The column target list holds column fields only, that are retrieved from one base relation / view only. (NO joined
views).
- UNION [ALL]/EXCEPT, DISTINCT and GROUP BY query expressions aren't updateable at all.
- HAVING, Aggregates, function expressions and Subqueries aren't allowed to be updateable, too
NOTE: one option is add a catalog that contains info about updateability of the view attributes, just like
ORACLE'suser_updateable_column view (actually pg_attribute says what columns has a view, can it be
extended?). That way we can have views in which some columns
are updateable and other are not. Views with more complicated querys (even joined ones) can be allowed
thisway.
Insertable???
----------------------
We need to provide, at least, a value for every column
in the underlaying table that is NOT NULL and do not
have a DEFAULT value.
- If primary key of the table is a serial we can manage it CREATE RULE "ins_people_full" as ON INSERT TO people_full
DOINSTEAD
( INSERT INTO people (person_id, inits, fname) VALUES (nextval('people_person_id_seq'),NEW.inits,
NEW.fname); INSERT INTO addresses (person_id,city, state, zip) VALUES (currval('people_person_id_seq'), NEW.city,
NEW.state, NEW.zip);
);
- What if we add a new not null column without a default value to the underlaying table? The insert rule must be
deleted?
Updateable???
----------------------
Deleteable???
----------------------
- Can we delete a row from the underlaying table if the view where i execute the delete stmnt does not view all the
columnsin that table?
- What about joined views? What is deleted? Consider: CREATE VIEW people_full AS SELECT p.*, a.city, a.state,
s.state_long, a.country, a.zip FROM people p JOIN addresses a USING (person_id) JOIN states s USING
(state);
The a.city, a.state, s.state_long, a.country, a.zip columns must be deleted as well as the p.* columns
***********************************
- Other point is: some people will not be happy with updateable views, they will want their views to
be read-only. Should we have an extension to the sql
specs for this? Something like a READONLY keyword?
The patch Bernd did, actually covers some of this
points but is just for *very, very* simple views. We
want improve it.
These of course are just general ideas, and we really
want to know your opinion.
regards,
Jaime Casanova
_________________________________________________________
Do You Yahoo!?
Información de Estados Unidos y América Latina, en Yahoo! Noticias.
Visítanos en http://noticias.espanol.yahoo.com
> - What if we cannot create one of the three rules? > Make the rule not updateable at all? > Or create the rules we can? (i think this is the > correct) I seem to be in the minority here. But I think creating complex rules to fiddle with the updates to translate them to the underlying tables is the wrong approach. I think you want to extend the SQL syntax to allow updating views, and implement plan nodes and executor functionality to handle them. So things like this works: UPDATE (SELECT id,val FROM t) SET val=0 where id < 100 Then the rules you create on the views are just like the rules for SELECT, they simply mechanically replace the view with the view definition. I think this is the right approach because: a) I think creating the general rules to transform an update into an update on the underlying table will be extremely complex,and you'll only ever be able to handle the simplest cases. By handling the view at planning time you'll be ableto handle arbitrarily complex cases limited only by whether you can come up with reasonable semantics. b) I think it's aesthetically weird to have functionality that's only accessible via creating DDL objects and then usingthem, and not accessible directly in a single SQL DML command. Ie, it would be strange to have to create a "temporaryview" just in order to execute an update because there's no equivalent syntax available for use directly. > General Restrictions!!! > --------------------------- > - The column target list holds column fields only, > that are retrieved from one base relation / view > only. (NO joined views). I know there are other uses for updatable views (eg implementing column-based security policies) but the _only_ reason I ever found them useful in Oracle was precisely for joined views. They're the Oracle blessed method for achieving the same performance win as Postgres's FROM clause. So in Oracle you can do: UPDATE (select a.val as newval, b.b_id, b.val from a,b where a.b_id = b.b_id) SET val = newval -- greg
--- Greg Stark <gsstark@mit.edu> escribió: > > > - What if we cannot create one of the three > > rules? > > Make the rule not updateable at all? > > Or create the rules we can? (i think this is > > the correct) > > I seem to be in the minority here. But I think > creating complex rules to fiddle with the updates > to translate them to the underlying tables is the > wrong approach. > > I think you want to extend the SQL syntax to allow > updating views, and implement plan nodes and > executor functionality to handle them. What if someone want his views to be readonly? with rules he can just drop rule. In the approach you mention he cannot. > So things like this works: > > UPDATE (SELECT id,val FROM t) SET val=0 where id < > 100 > ???? You really do things like that??? For what?? I'm asking because i do not know any situation when it becomes usefull. Views, conceptually, should have the same behavior a table has, because you can use it to let some people view part of your info without letting them touch the table. Sometimes you need they can update the fields they can see, but then how u can prevent them touching other fields they have no rights to? Updateable views are handy for that. In your example is obvious that you can access to the t table, why not do the update directly?? Besides, this enforce to create privileges per columns rather than per table. > Then the rules you create on the views are just like > the rules for SELECT, they simply mechanically > replace the view with the view definition. > > I think this is the right approach because: > > a) I think creating the general rules to transform > an update into an update on the underlying table > will be extremely complex, and you'll only ever be > able to handle the simplest cases. By handling > the view at planning time you'll be able to > handle arbitrarily complex cases limited only by > whether you can come up with reasonable semantics. > I don't think is *extremely complex* to create the rules; but yes, there will be limitations. > b) I think it's aesthetically weird to have > functionality that's only accessible via creating > DDL objects and then using them, and not > accessible directly in a single SQL DML command. > Ie, it would be strange to have to create > a "temporary view" just in order to execute an > update because there's no equivalent syntax > available for use directly. > ???? alter table (SELECT id,val FROM t) alter column val set default 3; ???? > > General Restrictions!!! > > --------------------------- > > - The column target list holds column fields only, > > that are retrieved from one base relation / view > > only. (NO joined views). > > I know there are other uses for updatable views (eg > implementing column-based security policies) but the > _only_ reason I ever found them useful in Oracle > was precisely for joined views. The NOTE i included in my last post says that oracle do that with user_updateable_columns view and i suggest the creation (or the extension of pg_attribute) of a catalog to implement this. And i state that can be useful to create joined updateable views. > They're the Oracle blessed method for achieving the > same performance win as Postgres's FROM clause. > > So in Oracle you can do: > > UPDATE (select a.val as newval, b.b_id, b.val from > a,b where a.b_id = b.b_id) SET val = newval > I think Postgres's UPDATE ... FROM is a lot more clear to understand. regards, Jaime Casanova _________________________________________________________ Do You Yahoo!? Información de Estados Unidos y América Latina, en Yahoo! Noticias. Visítanos en http://noticias.espanol.yahoo.com
Greg Stark <gsstark@mit.edu> writes:
> I think you want to extend the SQL syntax to allow updating views, and
> implement plan nodes and executor functionality to handle them. So things
> like this works:
> UPDATE (SELECT id,val FROM t) SET val=0 where id < 100
> Then the rules you create on the views are just like the rules for SELECT,
> they simply mechanically replace the view with the view definition.
> I think this is the right approach because:
> a) I think creating the general rules to transform an update into an update on
> the underlying table will be extremely complex, and you'll only ever be
> able to handle the simplest cases. By handling the view at planning time
> you'll be able to handle arbitrarily complex cases limited only by whether
> you can come up with reasonable semantics.
Please provide an existence proof. I don't really see any basis for the
claim that this will be simpler to implement --- the semantic problems
will be the same either way.
regards, tom lane