Re: Oracle Style packages on postgres

OK, so it seems we need:

o make private objects accessable only to objects
in the same schema
o Allow current_schema.objname to access current
schema objects
o session variables
o nested schemas?

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Dave Held wrote:
> > -----Original Message-----
> > From: Bruce Momjian [mailto:pgman(at)candle(dot)pha(dot)pa(dot)us]
> > Sent: Tuesday, May 10, 2005 8:43 AM
> > To: Thomas Hallgren
> > Cc: Tom Lane; rmm(at)sqlisor(dot)com; pgsql-hackers(at)postgresql(dot)org
> > Subject: Re: [HACKERS] Oracle Style packages on postgres
> >
> > [...]
> > I suppose. I think we should focus on the use cases for Oracle
> > packages, rather than the specific functionality it provides.
> > What things do people need PostgreSQL to do that it already
> > doesn't do?
>
> Is that really the best way to go about things? Already RDBMSes
> are patchwork quilts of functionality. Is merely adding another
> patch the most elegant way to evolve the database? The problem is
> that Oracle et al are trying to be ORDBMSes and aren't exactly sure
> what the best way to go is. Instead of trying to formulate a
> rational plan for what an ORDBMS should even look like, they simply
> look at what would work with their existing infrastructure and tack
> on features. Then Postgres plays the copycat game. Instead of
> trying to play catch-up with Oracle, why not beat them at their own
> game?
>
> What packages provide is encapsulation. Hiding the data from the
> user and forcing him/her to use the public interface (methods).
> That is an important and admirable OO feature. Some people think
> that using the DB's security model can achieve the same thing. It
> can't, exactly, but there's an important lesson to be learned from
> the suggestion. The problem is that OOP is a *programming* paradigm,
> and a database is not a *programming language*. In a programming
> language, there really is no such thing as "security". There is
> only "visibility" and "accessibility". Private methods in an OOP
> language do not provide *security*; they only limit *accessibility*.
> Like so many other differences between the relational model and the
> OOP model, there is an impedance mismatch here. However, there is
> also opportunity.
>
> In an OOPL, you can say: "Users can call this method from here, but
> not from there." What you *can't* say is: "User X can call this
> method, but User Y cannot." As you can see, these are orthogonal
> concepts. You could call the first "accessibility by location" and
> the second "accessibility by authentication". An ORDBMS should
> support both. "Private" does not respect your identity, only your
> calling location. An ACL does not respect your calling scope, only
> your identity. A system that has both is clearly more flexible than
> one that only has one or the other.
>
> Now what you need to keep in mind is that each visibility model
> serves a different purpose. The purpose of a security model is to
> limit *who* can see/touch certain data because the data has intrinsic
> value. The purpose of an accessibility model is to limit *where* and
> *how* data can be seen/touched in order to preserve *program
> invariants*. So if you have an object (or tuple!) that records the
> start and stop time of some process, it is probably a logical
> invariant that the stop time is greater than or equal to the start
> time. For this reason, in a PL, you would encapsulate these fields
> (attributes) and only provide controlled access to update them that
> checks and preserves the invariant, *no matter who you are*. You
> don't want a superuser violating this invariant any more than Sue
> User.
>
> Now you might object that constraints allow you to preserve
> invariants as well, and indeed they do. But constraints do not
> respect calling scope. Suppose there is a process that needs to
> update the timestamps in a way that temporarily breaks the invariant
> but restores it afterwards. The only way to effect this in a
> constraint environment is to drop the constraint, perform the
> operation, and restore it. However, dropping a constraint is not an
> ideal solution because there may be other unprivileged processes
> operating on the relation that still need the constraint to be
> enforced. There is no way to say: "There is a priviledged class of
> methods that is allowed to violate this constraint because they are
> trusted to restore it upon completion." Note that this is different
> from saying "There is a priviledged class of users that is allowed
> to violate this constraint." If you try to do something like give
> read-only access to everybody and only write access to one user and
> define that user to be the owner of the methods that update the data,
> you have to follow the convention that that user only operates
> through the defined interface, and doesn't hack the data directly.
> That's because user-level accessibility is not the same as scope-
> level accessibility. Whereas, if you define something like a
> package, and say: "Package X is allowed full and complete access
> to relation Y", and stick the interface methods in X, you still have
> all the user-level security you want while preserving the invariants
> in the most elegant way.
>
> So you can think of a package as a scope in a programming language.
> It's like a user, but it is not a user. A user has privileges that
> cut across scopes. Now, whether packages should be different from
> schemas is a whole different ballgame. The purpose of a schema in
> Postgres is not entirely clear to me. There's lots of different ways
> to use schemas, and there is no obvious best way to use them. In
> order to implement the accessibility features of packages, schemas
> would have to be changed considerably. Probably a lot of users would
> be unhappy if schemas were changed in that way. My guess is that
> this would not be a good idea.
>
> I think we can get some guidance from PLs. C++ is what you call a
> "multi-paradigm language". You can do everything from assembly to
> metaprogramming in C++. As such, it is very loose and open in some
> respects. C++ has two kinds of scopes: it has classes and namespaces.
> Members of a class are encapsulated and support data hiding. Members
> of a namespace are only loosely grouped and do not support data hiding
> explicitly. Namespaces exist primarily to avoid name collisions.
>
> Java, on the other hand, decided that for OOP purity, everything must
> be a class. That would be like making schemas into packages and
> imposing accessibility rules on them. At the end of the day, I think
> many PL design experts agree that making everything a class is not
> necessarily the best way to go.
>
> So schemas can be like C++ namespaces - they provide a means to
> loosely group related objects and help avoid name collisions. So
> the package could be like a class - they provide OOP-like
> encapsulation via accessibility rules. However, that doesn't mean
> that nested schemas wouldn't also be a good thing. In C++, nested
> namespaces are extremely useful when one layer of scoping does not
> sufficiently partition the namespace to avoid frequent name
> collisions. I think the same is true of Postgres. I certainly would
> like to be able to use nested schema names in several contexts.
> Instead, I have to make a choice between making different schemas,
> or making different name prefixes. I wouldn't even mind if nested
> schemas were only allowed to contain schemas except at the leaves of
> the tree. Another feature that is very useful is the "using clause".
> Combined with nested namespaces, this is a very powerful way to give
> programmers/dbas control over names. You can give everything the
> most natural name, and just put it in the appropriate namespace,
> and use the namespace that is relevant to the given task at hand.
>
> So consider this example:
>
> Tables:
> etl.import.record
> etl.export.record
>
> As you can imagine, I don't really want to make an 'import' and
> 'export' schema at the top level. There's several tables in
> each schema, but that should illustrate the point. Then, when
> constructing queries, it would be nice to be able to do this:
>
> USING etl.import
> ;
> SELECT *
> FROM record
> JOIN header ON ...
> JOIN file ON ...
> ;
>
> The effect of a USING clause would be to import the schema names
> into the public namespace for the duration of the transaction. If
> that leads to ambiguous names, then the parser/planner should emit an
> error.
>
> __
> David B. Held
> Software Engineer/Array Services Group
> 200 14th Ave. East, Sartell, MN 56377
> 320.534.3637 320.253.7800 800.752.8129
>
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