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To: Bruce Momjian <pgman@candle.pha.pa.us>

cc: Stephan Szabo <sszabo@megazone23.bigpanda.com>,

PostgreSQL-development <pgsql-hackers@postgresql.org>

Subject: Re: [HACKERS] Re: [GENERAL] child table doesn't inherit PRIMARY KEY?

In-reply-to: Message from Bruce Momjian <pgman@candle.pha.pa.us>

of Wed, 24 Jan 2001 14:31:29 EST. <200101241931.OAA26463@candle.pha.pa.us>

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Date: Wed, 24 Jan 2001 21:41:39 +0000

From: "Oliver Elphick" <olly@lfix.co.uk>

Status: OR

Bruce Momjian wrote:

>> On Wed, 24 Jan 2001, Bruce Momjian wrote:

>I smell TODO item. In fact, I now see a TODO item:

>

>* Unique index on base column not honored on inserts from inherited table

> INSERT INTO inherit_table (unique_index_col) VALUES (dup) should fail

> [inherit]

>

>So it seems the fact the UNIQUE doesn't apply to the new table is just a

>manifestion of the fact that people expect UNIQUE to span the entire

>inheritance tree. I will add the emails to [inherit] and mark it as

>resolved.

Bruce, could you add this text to TODO.detail on the subject of

inherited constraints. I first sent it on Christmas Eve, and I

think most people were too busy holidaying to comment.

=================================================================

Tom Lane wrote:

>Hm. The short-term answer seems to be to modify the queries generated

>by the RI triggers to say "ONLY foo". I am not sure whether we

>understand the semantics involved in allowing a REFERENCES target to be

>taken as an inheritance tree rather than just one table, but certainly

>the current implementation won't handle that correctly.

May I propose these semantics as a basis for future development:

1. An inheritance hierarchy (starting at any point in a tree) should be

equivalent to an updatable view of all the tables at the point of

reference and below. By default, all descendant tables are combined

with the ancestor for all purposes. The keyword ONLY must be used to

alter this behaviour. Only inherited columns of descendant tables are

visible from higher in the tree. Columns may not be dropped in descendants.

If columns are added to ancestors, they must be inserted correctly in

descendants so as to preserve column ordering and inheritance. If

a column is dropped in an ancestor, it is dropped in all descendants.

2. Insertion into a hierarchy means insertion into the table named in

the INSERT statement; updating or deletion affects whichever table(s)

the affected rows are found in. Updating cannot move a row from one

table to another.

3. Inheritance of a table implies inheriting all its constraints unless

ONLY is used or the constraints are subsequently dropped; again, dropping

operates through all descendant tables. A primary key, foreign key or

unique constraint cannot be dropped or modified for a descendant. A

unique index on a column is shared by all tables below the table for

which it is declared. It cannot be dropped for any descendant.

In other words, only NOT NULL and CHECK constraints can be dropped in

descendants.

In multiple inheritance, a column may inherit multiple unique indices

from its several ancestors. All inherited constraints must be satisfied

together (though check constraints may be dropped).

4. RI to a table implies the inclusion of all its descendants in the

check. Since a referenced column may be uniquely indexed further up

the hierarchy than in the table named, the check must ensure that

the referenced value occurs in the right segment of the hierarchy. RI

to one particular level of the hierarchy, excluding descendants, requires

the use of ONLY in the constraint.

5. Dropping a table implies dropping all its descendants.

6. Changes of permissions on a table propagate to all its descendants.

Permissions on descendants may be looser than those on ancestors; they

may not be more restrictive.

This scheme is a lot more restrictive than C++'s or Eiffel's definition

of inheritance, but it seems to me to make the concept truly useful,

without introducing excessive complexity.

============================================================

--

Oliver Elphick Oliver.Elphick@lfix.co.uk

Isle of Wight http://www.lfix.co.uk/oliver

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========================================

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