Once again, many many thanks Jeff for taking the time to think
through these issues and provide your well-informed comments & opinions!
On Aug 10, 2005, at 4:09 PM, Jeff Davis wrote:
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,
a point well taken. I can remember how excited I was when Oracle
first started providing object extensions (which provides richer
capabilities than INHERITs currently does). Of course, that was a
good decade after PostgreSQL/post-Ingres had added INHERITs (http://
en.wikipedia.org/wiki/PostgreSQL).
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>
Many thanks for this link. I had grown quite addicted to
Materialized Views in Oracle, especially when working on OLAP
applications. Though I've been a very happy convert to PostgreSQL
(for about 2 years ago), I've really missed having MATERIALIZED VIEWs
integrated into the core DDL SQL of the RDBMS.
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.
Both are excellent points. I will certainly check the bug lists
before getting too committed to using INHERITs, though, as you say,
hopefully the implementation relies on other components in the system
getting heavy use (and testing).
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.
I'm a VERY STRONG believer in keeping application requirements out of
the logical data model - probably too much so sometimes. In general,
it has rarely served me wrong, when I've needed to go in and write a
wholly separate application to the same underlying data. I really
appreciate your placing INHERITs in this context. It would have
completely slipped by me, though it should have been obvious.
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 really like your breakdown here. You are absolutely right. I most
definitely meant "late binding", not "loosely typed". As a very
heavy user of Ruby - having received many a compiler error just like
the one you site - I should have remembered that distinction. In
fact, it's Ruby's class typing that has made it such a powerful tool
for the work I do. It has also enabled me to take scripts I quickly
work up in Ruby and port them straight to Java (where the bulk of my
production code lives) with relative ease.
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?)?
All excellent, subtle questions that will greatly effect the
implementation flexibility INHERITs provides.
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.
I'm not certain I understand what you mean here? Are you
recommending all application layer interaction with tables using
INHERIT should be done via a VIEW intermediary? If so, wouldn't the
VIEW (built from a "SELECT ... ONLY...") then be as dependent on the
fixed structure determined by the INHERITs relationship, as much as
the application code would be?
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".
Absolutely, yes. Since I need to wear several hats - DBA, database
programmer, Java GUI App architect/implementer - anything I can do to
maximize my efficiency in any one role helps to ensure I have at
least a little of my weekend time to myself. :-)
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.
Sorry - you are right, of course. I'm used to the Oracle
implementation of TABLESPACEs, where these two separate issues are
somewhat convolved together.
Regards,
Jeff Davis
---------------------------(end of
broadcast)---------------------------
TIP 9: In versions below 8.0, the planner will ignore your desire to
choose an index scan if your joining column's datatypes do not
match
---------------------------(end of broadcast)---------------------------
TIP 6: explain analyze is your friend
