Thread: Re: Why is MySQL more chosen over PostgreSQL
Curt Sampson wrote: > I'm still waiting to find out just what advantage table inheritance > offers. I've asked a couple of times here, and nobody has even started > to come up with anything. Table inheritance offers data model extensibility. New (derived) tables can be added to the system, and will work with existing code that opperates on the base tables, without having to hack up all the code. Inherited indexes etc. would be nice, but it's the inability to have referential integrity against a base table that picks up child table rows that makes the current implementation useles. I would rather see it fixed than junked, and better yet extended. It would be incredibly useful in real-world projects with complex data models like OpenACS.
On 29 Jul 2002 18:27:40 MDT, the world broke into rejoicing as Stephen Deasey <stephen@bollocks.net> said: > Curt Sampson wrote: >> I'm still waiting to find out just what advantage table inheritance >> offers. I've asked a couple of times here, and nobody has even >> started to come up with anything. > Table inheritance offers data model extensibility. New (derived) tables > can be added to the system, and will work with existing code that > operates on the base tables, without having to hack up all the code. But it kind of begs the question of why you're creating the new table in the first place. The new table certainly _won't_ work with existing code, at least from the perspective that the existing code doesn't _refer_ to that table. The same is not true for views; if you create a new view on a table, the existing code that refers to the table in either "raw" form or in other views already exist will certainly continue to work. > Inherited indexes etc. would be nice, but it's the inability to have > referential integrity against a base table that picks up child table > rows that makes the current implementation useles. Views will certainly inherit indices, and continue to maintain referential integrity against the "base" table. > I would rather see it fixed than junked, and better yet extended. It > would be incredibly useful in real-world projects with complex data > models like OpenACS. Have they found views to be an unacceptable alternative? I just don't see that there's _all_ that much about table inheritance that is really fundamentally wonderful. The "incredibly useful" thing, it seems to me, would be to provide tools that make it less necessary to create additional tables. To be sure, views do that. I'm not quite sure what INHERITS buys us that we don't get from SELECT * into new_table from old_table INHERITS may automagically draw in some contraints that have been specifically tied to old_table that wouldn't be drawn by "SELECT * INTO NEW_TABLE"; the latter _will_ inherit anything that is tied to the types. I'd _much_ rather have five views on one table than five tables. -- (reverse (concatenate 'string "gro.gultn@" "enworbbc")) http://cbbrowne.com/info/spreadsheets.html "Everything should be made as simple as possible, but not simpler." -- Albert Einstein
On Fri, 2002-08-02 at 22:39, cbbrowne@cbbrowne.com wrote:
> On 29 Jul 2002 18:27:40 MDT, the world broke into rejoicing as
> Stephen Deasey <stephen@bollocks.net> said:
> > Curt Sampson wrote:
> >> I'm still waiting to find out just what advantage table inheritance
> >> offers. I've asked a couple of times here, and nobody has even
> >> started to come up with anything.
>
> > Table inheritance offers data model extensibility. New (derived) tables
> > can be added to the system, and will work with existing code that
> > operates on the base tables, without having to hack up all the code.
>
> But it kind of begs the question of why you're creating the new table in
> the first place.
>
> The new table certainly _won't_ work with existing code, at least from
> the perspective that the existing code doesn't _refer_ to that table.
The beuty of OO is that it does not need to :
hannu=# create table animal (name text, legcount int);
CREATE
hannu=# insert into animal values('pig',4);
INSERT 34183 1
hannu=# select * from animal;name | legcount
------+----------pig | 4
(1 row)
hannu=# create table bird (wingcount int) inherits (animal);
CREATE
hannu=# insert into bird values('hen',2,2);
INSERT 34189 1
hannu=# select * from animal;name | legcount
------+----------pig | 4hen | 2
(2 rows)
------------------
Hannu
> On Fri, 2002-08-02 at 22:39, cbbrowne@cbbrowne.com wrote: > >>On 29 Jul 2002 18:27:40 MDT, the world broke into rejoicing as >>Stephen Deasey <stephen@bollocks.net> said: >> >>>Curt Sampson wrote: >>> >>>>I'm still waiting to find out just what advantage table inheritance >>>>offers. I've asked a couple of times here, and nobody has even >>>>started to come up with anything. >>> >>>Table inheritance offers data model extensibility. New (derived) tables >>>can be added to the system, and will work with existing code that >>>operates on the base tables, without having to hack up all the code. >> >>But it kind of begs the question of why you're creating the new table in >>the first place. >> >>The new table certainly _won't_ work with existing code, at least from >>the perspective that the existing code doesn't _refer_ to that table. Since OpenACS has been brought up in this thread, I thought I'd join the list for a day or two and offer my perspective as the project manager. 1. Yes, we use views in our quasi-object oriented data model. They're automatically generated when content types are built by the content repository, for instance. 2. Yes, you can model anything you can model with PG's OO extensions using views. If you haven't implemented some way to generate the view automatically then a bit more work is required compared to using PG's OO extensions. 3. The view approach requires joins on all the subtype tables. If I declare type 'foo' then the view that returns all of foo's columns joins on all the subtype tables, while in the PG OO case all of foo's columns are stored in foo meaning I can get them all back with a simple query on the table. The PG OO approach can be considerably more efficient than the view approach, and this is important to some folks, no matter how many appeals to authority are made to various bibles on relational theory written by Date and Darwen. 4. The killer that makes the current implementation unusable for us is the fact that there's no form of indexing that spans all the tables inherited from a base type. This means there's no cheap enforcement of uniqueness constraints across a set of object types, among other things. Being able to inherit indexes and constraints wouldgreatly increase the utility of PG's OO extensions. 5. If PG's OO extensions included inheritance of indexes and constraints, there's no doubt we'd use them in the OpenACS project, because when researching PG we compared datamodels written in this style vs. modelling the object relationships manually with automatically generated views. We found the datamodel written using PG's OO extensions not only potentially more efficient, but more readable as well. As far as whether or not there's a significant maintenance cost associated with keeping the existing OO stuff in PG, Tom Lane's voice is authorative while, when it comes to PG internals, Curt Sampson doesn't know squat. -- Don Baccus Portland, OR http://donb.photo.net, http://birdnotes.net, http://openacs.org
On 29 Jul 2002, Stephen Deasey wrote: > Table inheritance offers data model extensibility. New (derived) tables > can be added to the system, and will work with existing code that > opperates on the base tables, without having to hack up all the code. And why does this not work with the standard relational mechanism? (Where a "derived table" would be just another table with a foreign key pointing back to the base table.) cjs -- Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org Don't you know, in this new Dark Age, we're alllight. --XTC
On 3 Aug 2002, Hannu Krosing wrote:
> hannu=# create table animal (name text, legcount int);
> CREATE
> hannu=# insert into animal values('pig',4);
> INSERT 34183 1
> hannu=# select * from animal;
> name | legcount
> ------+----------
> pig | 4
> (1 row)
>
> hannu=# create table bird (wingcount int) inherits (animal);
> CREATE
> hannu=# insert into bird values('hen',2,2);
> INSERT 34189 1
> hannu=# select * from animal;
> name | legcount
> ------+----------
> pig | 4
> hen | 2
> (2 rows)
You can do this just as well with views. In posgres, it's harder
only because you're forced to create rules for updating views. But
it's possible to have the system automatically do the right thing.
cjs
--
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org Don't you know, in this new Dark Age, we're
alllight. --XTC