Re: Inheritance

At 08:33 AM 9/7/02, Oliver Elphick wrote:
>On Fri, 2002-09-06 at 19:00, elein wrote:
> >
> >
> > There was a comment earlier that was not really addressed.
> > What can you do with table inheritance that you can not do
> > with a relational implementation?  Or what would work *better*
> > as inheritance?  (you define better)
>
>There is nothing that you cannot do in some way; that way may not be
>very convenient compared to the use of inheritance.  I consider
>simplicity to be preferable to conceptual purity.

yes, simplicity is a very reasonable criteria for better.

> > This is a genuine question, not a snarky comment.  I really
> > want to know.  This is the reason I can think of to use
> > inheritance: Several tables have a common set of attributes and
> > there is some reason for these tables to be separate AND there
> > is some reason for the common columns to be queried en masse.
> > What kinds of "some reasons" are there, though?  And if my
> > condition for using table inheritance is lacking or misguided, what should
> > be the criteria for using table inheritance?

In non-OO terms, you have both reasons for tables to
be separate and reasons to query an entire hierarchy.
Your exact reasons are clear and reasonable.
This is helpful.

>I use it when a group of tables are closely related; they are all
>members of some higher class.  For example:
>
>                                person <.......................> address
>                                   |
>                    +--------------+--------------+
>                    |                             |
>              organisation                   individual <......> pay_tax
>                    |                             |
>           +--------+--------+          +---------+---------+
>           |        |        |          |         |         |
>      customer  supplier ...etc...   staff  homeworker ...etc...
>           |
>      +----+-------------+
>      |                  |
>home_customer    export_customer
>
>It is convenient to use a higher class when you are interested in all
>its members and only in the attributes of the higher class.  So I can
>say
>
>    SELECT * FROM person,address
>     WHERE address.person = person.id AND
>           address.town = 'London';
>
>to get all rows for people in London.  I will only get those attributes
>that are in person itself; if I want to know about credit limits, that
>is only relevant in the customer hierarchy and I have to SELECT from
>customer instead..
>
>Similarly, I can use the whole customer hierarchy when changing or
>reporting on outstanding customer balances.

I don't think table inheritance will "go away" and so being
consistent about the indexes and constraints is necessary
in order to keep its usage simpler.  This might lessen the PITA
factor for a few more people, but we should prioritize the
project.  I think few people have put the thought into it that
you have.


>If foreign key relations were valid against an inheritance tree, I could
>implement it for a table of addresses referencing the highest level
>(every person has an address) and of pay and tax records at the
>individual level.  These don't change as you go down the hierarchy, but
>a purely relational implementation has to be redone at each level.  A
>reciprocal relation requires an extra table to hold all the hierarchy's
>keys and that in turn needs triggers to keep that table maintained.
>(I.e., person should have a FK reference to address and address to
>person; instead, address needs a reference to person_keys, which I have
>to create because FK against a hierarchy isn't valid.)  The lack of
>inherited RI makes the design more complex and more difficult to
>understand.
>
>--
>Oliver Elphick                                Oliver.Elphick@lfix.co.uk
>Isle of Wight, UK
>http://www.lfix.co.uk/oliver
>GPG: 1024D/3E1D0C1C: CA12 09E0 E8D5 8870 5839  932A 614D 4C34 3E1D 0C1C
>                  ========================================
>      "For whosoever shall call upon the name of the Lord
>       shall be saved."         Romans 10:13
>
>
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:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:~:    elein@norcov.com   (510)543-6079    "Taking a Trip. Not taking a
Trip."--anonymous
 
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