Re: Inheritance vs. LIKE - need advice

William Bug wrote:
> As you say, both LIKE & INHERIT are a bit anemic and unnecessary, since
> what they provide can be implemented via VIEWs and RULEs/ TRIGGERs. I'd

I'd like to point out that INHERITS is unique, although I'm not sure all
of the exact differences. The main difference that I see is that SELECT
reads from multiple tables, and to make it only read one you do SELECT
... ONLY.

LIKE is purely syntactic sugar. Use it whenever it saves time and/or
reduces confusion.

> VIEWs, at least). Being able to create MATERIALIZED VIEWs would
> probably help to remove any realtime performance issues introduced by
> requiring additional JOINs to harvest this view of the data. I can do
> this myself via RULEs/ TRIGGERs or use the system being developed as an
> extension to Pg - The matview Project
> (http://gborg.postgresql.org/project/matview/ projdisplay.php).

Agreed. Materialized views are a great way to get whatever performance
benefits you need from physical representation without imposing on the
logical layout.

For some reason a good document on the subject is on a server that's not
responding right now (hopefully temporarily!). Here's the google cache:
<http://66.102.7.104/search?q=cache:jaaXngt0hioJ:jonathangardner.net/PostgreSQL/materialized_views/matviews.html+materialized+view+postgresql&hl=en>

> I actually think the INHERITs & LIKE features of PostgreSQL - in their
> simplicity - potentially offer a better means to providing Object
> properties to SQL relations, than the more complex, fully realized
> Object-Relational systems, which are inherently better suited to the
> task of providing a seamless persistence mechanism for OOP code
> objects. I do hope its not true Pg's INHERIT & LIKE features are
> vestigial and will continue to be developed into the future. in the

I think I overstated what I meant in my previous email... it should be
more like "it's in a stasis" rather than "on life support". When PG
solves some of the table partitioning issues in future releases, you can
bet that those features will help complete the inheritance model. After
a while, INHERITS will also be merely a syntax for capabilities that are
available otherwise.

> meantime, in light of what you say regarding the lack of active
> development and maintenance they are receiving, I'll probably lay off
> using them much for now. As you say, heavy use of INHERITs given the
> current support given to this feature is more a liability than a
> convenience at this point.

Not too many people use INHERITS. I think it's fairly independent in the
code and probably not too many bugs appear, but if a bug creeps in, the
limited testing might not catch it. Maybe a brief scan of the -bugs list
might indicate whether inheritance is a source of problems, or benign
syntax that primarily depends on other database features which are
well-tested.

>
> I wasn't clear about this in that initial post, but my references to
> OOP techniques were just by way of analogy. I wasn't really asking the
> question with a thought toward creating a model to mirror my OOP models
> - to be simply a persistence mechanism. There are many wonderful

It was mainly just a warning that sometimes application algorithms tend
to mix with the data model.

> So - the real point I was trying to make is:
> 1) INHERIT & OOP inheritance: each provide a relatively efficient
> means to model real-world objects with inherent parent- child,
> hierarchical relations to one another;

True, many real-world entities have the famous "isa" relationship, and
that's valueable to take into account in the data model.

> 2) LIKE & Interfaces(Java)/Mixins(Ruby): each provide a means to
> share a set of pre-defined attributes/methods amongst a set of class
> that otherwise have no inherent hierarchical relation to one another.
>
> You are certainly correct - Interfaces (Java) are essentially a
> compile-time enforcement mechanism. Changes you make to an Interface
> after it has been used to generate bytecodes (i.e., compile) for a Java
> class will not have any effect on the class definition until you
> compile it again. Interfaces provide a fairly simple - but effective -
> means to an end - for Java to provide for "inheriting" aspects from
> more than one existing code entity without supporting multiple
> inheritance (which Pg actually does support). Mixins in Ruby, on the
> other hand, don't really get added to a class until runtime (though
> they are used at compile time to resolve function & variable calls).
> If you add to a Mixin, you could actually use that new feature next
> time you use a class whose definition file included that Mixin. This
> is largely due to the fact that Ruby is interpreted at runtime and is a
> very loosely typed language.

Definitely a tangent, but I think most people would consider Ruby
strongly typed. Consider:

$ ruby -e 'puts 1+"1"'
-e:1:in `+': String can't be coerced into Fixnum (TypeError)
from -e:1
$ perl -e 'print 1+"1","\n";'
2

However, ruby is, like python, late-binding. That means a variable can
take on a value of any type, but it gets the type with the value. I
break it down kind of like this: Ruby and Python are strongly typed and
late binding; perl and PHP are weakly typed and late binding; C is
weakly typed and early binding; Java is strongly typed and early binding
(that isn't entirely true... dynamic binding is sort of like late
binding). That's not official or anything, just my opinion of the
languages that I use.

>
> I don't think I was very clear about this, but I was asking these
> questions about Pg's schema reusability mechanisms solely from the
> vantage of how it might save me time in writing and maintaining the DDL
> SQL for my core relational model. I was also interested in how the
> INHERITANCE mechanism might make certain SQL operations easier to
> implement in the case of where you have real-world objects you are
> modeling which possess an inherent hierarchical relation to one
> another. Finally, I wanted to know whether my thinking regarding when
> to use LIKE over INHERIT - and visa versa - was correct given their
> current implementation in Pg.

I think you have the right idea about LIKE and INHERITS. I thought your
comment about how you would be unlikely to want to query "curatable
entities" was an insightful way of looking at it.

I think overall, the most important thing is flexibility. I always think
to myself: how much work will it take if I wanted to change, or add
applications accessing the same data set?

If your applications have "SELECT ... ONLY" in them, what effect will
that have on your ability to change the physical layout (I actually
don't know the answer to that. If you create a view on some relations,
whether they're parent relations or not, and you "SELECT ... ONLY" on
the view, is the ONLY ignored? Can views resemble a part of an
inheritance hierarchy?)?

I would recommend that if you use INHERITS, always have a view in
between so that your application aren't tied to the physical layout in
any way. Because a view is not treated like a physical table in
inheritance (you can't INHERIT a view), it's too closely tied to the
physical layout.

> When I say "LIKE" offers some efficiencies, I mean just what I say
> above. For instance, with the example I gave previously, if I have a
> set of fields - curation_info (creation_curator TEXT, creation_date
> TIMESTAMP, mod_curator TEXT, mod_date TIMESTAMP) - I want represented
> in all 100 tables in my model, I save a great deal of SQL writing by
> creating a curation_info table by including it in these tables via
> LIKE. The fact the LIKE included table is in no way linked to the
> tables that include it after execution of the initial CREATE TABLE
> statement makes this a bit less useful than it might otherwise be. If
> a change is made to the fields in the underlying 'curation_info' table,
> you are correct, I can always use ALTER TABLE to update the 100 tables
> that included the fields from 'curation_info' later. Since I can't use
> LIKE in a ALTER TABLE statement, however, I'd have to specify each
> field to be changed in the ALTER statement for each of the 100 tables.
> This forces me to write a lot more SQL than I'd like to write. Of
> course, to ask this of the "LIKE" function is to ask for more than you
> get from a Java Interface and maybe it would add unwanted
> inefficiencies to the query planner/execution framework.

LIKE actually fills in the fields at table creation time exactly as if
you wrote the field names out yourself, so I don't follow the
planner/execution comment. But I see what you're saying about adding
attributes if you have a lot of relations involved. Originally I was
unsure whether you meant "performance efficiency", but it's apparent you
mean "efficiency of DBA's time".

> Thanks very much for your insight on how the recent implementation of
> TABLESPACEs might lay the groundwork for adding new efficiencies to the
> INDEXing and inherited CONSTRAINT implementation for INHERITed tables.
> This would be a big plus.

Actually, I was talking about table partitioning, which is different
than a tablespace. A table space is a named storage "area" that you can
assign whole tables to (already implemented in 8.0). If you partition a
table, you break a table into pieces and those pieces can go to a
different place in physical storage. Partitioning is closely related to
INHERITS, because if they can implement constraints or keys across parts
of a table in different locations, they can do it for INHERITS also.

Regards,
Jeff Davis