Add inheritance.
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Bruce Momjian
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From: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
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To: "Hackers" <pgsql-hackers@postgresql.org>
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Subject: [HACKERS] Question about inheritance
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Date: Tue, 5 Jun 2001 09:42:38 +0800
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Message-ID: <ECEHIKNFIMMECLEBJFIGEENPCAAA.chriskl@familyhealth.com.au>
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Hi guys,
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It's relatively straightforward to allow check constraints to be inherited -
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but is it really possible to ever do the same with primary, unique or even
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foreign constraints?
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ie. Say a table has a primary key and I inherit from this table. Since the
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primary key is an index on the parent table, I could just create another
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index on the child table, on the same column.
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However - because we are dealing with two separate indices, it should still
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be possible to insert duplicate values into the parent table and the child
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table shouldn't it? This means that when a query is run over the parent
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table that includes results from the child table then you will get duplicate
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results in a supposedly primary index.
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Similar arguments seem to apply to unique and foreign constraints. If you
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could use aggregate functions in check constraints - you'd have another
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problem. And if asserts were ever implemented - same thing...
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Am I misunderstanding how the mechanism works, or is this a big, not easily
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solved, problem?
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Chris
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Mon, 4 Jun 2001 22:07:42 -0400 (EDT)
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To: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
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cc: "Hackers" <pgsql-hackers@postgresql.org>
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Subject: Re: [HACKERS] Question about inheritance
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In-Reply-To: <ECEHIKNFIMMECLEBJFIGEENPCAAA.chriskl@familyhealth.com.au>
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References: <ECEHIKNFIMMECLEBJFIGEENPCAAA.chriskl@familyhealth.com.au>
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Comments: In-reply-to "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
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message dated "Tue, 05 Jun 2001 09:42:38 +0800"
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Date: Mon, 04 Jun 2001 22:07:42 -0400
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Message-ID: <11249.991706862@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Status: OR
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"Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
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> Am I misunderstanding how the mechanism works, or is this a big, not easily
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> solved, problem?
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The latter. Check the list archives for previous debates about this.
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It's not real clear whether an inherited primary key should be expected
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to be unique across the whole inheritance tree, or only unique per-table
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(IIRC, plausible examples have been advanced for each case). If we want
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uniqueness across multiple tables, it'll take considerable work to
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create an index mechanism that'd enforce it.
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regards, tom lane
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Date: Tue, 5 Jun 2001 13:08:58 +1000
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Subject: Re: [HACKERS] Question about inheritance
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>It's relatively straightforward to allow check constraints to be inherited -
|
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>but is it really possible to ever do the same with primary, unique or even
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>foreign constraints?
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You would either have to check each index in the hierarchy or else have
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a single index across the whole hierarchy and check that. Obviously the
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latter would be generally more useful.
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As with all things inheritance, it is usually the right thing, and a good
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default that things be inherited. So ideally, indexes should work across
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whole hierarchies as well as primary, unique and foreign constraints.
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It could be argued that not inheriting is of very limited usefulness.
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From pgsql-hackers-owner+M9627@postgresql.org Mon Jun 4 23:58:36 2001
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Date: Mon, 4 Jun 2001 20:46:34 -0700 (PDT)
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From: Stephan Szabo <sszabo@megazone23.bigpanda.com>
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To: Christopher Kings-Lynne <chriskl@familyhealth.com.au>
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cc: Hackers <pgsql-hackers@postgresql.org>
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Subject: Re: [HACKERS] Question about inheritance
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In-Reply-To: <ECEHIKNFIMMECLEBJFIGEENPCAAA.chriskl@familyhealth.com.au>
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On Tue, 5 Jun 2001, Christopher Kings-Lynne wrote:
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|
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> Hi guys,
|
||||
>
|
||||
> It's relatively straightforward to allow check constraints to be inherited -
|
||||
> but is it really possible to ever do the same with primary, unique or even
|
||||
> foreign constraints?
|
||||
>
|
||||
> ie. Say a table has a primary key and I inherit from this table. Since the
|
||||
> primary key is an index on the parent table, I could just create another
|
||||
> index on the child table, on the same column.
|
||||
>
|
||||
> However - because we are dealing with two separate indices, it should still
|
||||
> be possible to insert duplicate values into the parent table and the child
|
||||
> table shouldn't it? This means that when a query is run over the parent
|
||||
> table that includes results from the child table then you will get duplicate
|
||||
> results in a supposedly primary index.
|
||||
>
|
||||
> Similar arguments seem to apply to unique and foreign constraints. If you
|
||||
> could use aggregate functions in check constraints - you'd have another
|
||||
> problem. And if asserts were ever implemented - same thing...
|
||||
>
|
||||
> Am I misunderstanding how the mechanism works, or is this a big, not easily
|
||||
> solved, problem?
|
||||
|
||||
It's a big deal. Actually check constraints have a similar problem if you
|
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allow inherited constraints to be dropped. "Why does 'select * from
|
||||
base;' give me rows where value<10 since there's a check value>=10
|
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on the table?"
|
||||
|
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As Tom said, the unique constraint thing is still questionable which is
|
||||
the more meaningful semantics. If we ever want to allow foreign key
|
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constraints to inheritance trees, we need *some* way to guarantees
|
||||
uniqueness across the tree even if that isn't through the unique
|
||||
constraint.
|
||||
|
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|
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From pgsql-hackers-owner+M9638@postgresql.org Tue Jun 5 06:30:37 2001
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To: <pgsql-hackers@postgresql.org>
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Subject: Re: [HACKERS] Question about inheritance
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> "Christopher Kings-Lynne" <chriskl@familyhealth.com.au> writes:
|
||||
> > Am I misunderstanding how the mechanism works, or is this a big, not
|
||||
easily
|
||||
> > solved, problem?
|
||||
>
|
||||
> The latter. Check the list archives for previous debates about this.
|
||||
> It's not real clear whether an inherited primary key should be expected
|
||||
> to be unique across the whole inheritance tree, or only unique per-table
|
||||
> (IIRC, plausible examples have been advanced for each case). If we want
|
||||
> uniqueness across multiple tables, it'll take considerable work to
|
||||
> create an index mechanism that'd enforce it.
|
||||
>
|
||||
IMHO current behaviour of PostgreSQL with inherited PK, FK, UNIQUE is
|
||||
simply
|
||||
bug not only from object-oriented but even object-related point of view.
|
||||
Now
|
||||
I can violate parent PK by inserting duplicate key in child!
|
||||
|
||||
Inherited tables should honours all constraints from parent. If I change
|
||||
some constraint (seems only FK, but not PK or UNIQUE) I should be able to
|
||||
do
|
||||
it in more restrictive manner. For example, two base table is connected via
|
||||
FK. I can change such FK in childs from base1->base2 to child1->child2 (or
|
||||
child3) but not to child1->not_inherited_from_base2. CHECK, DEFAULT, NOT
|
||||
NULL are more free to changes, isn't it?
|
||||
|
||||
IMHO last message in doc/TODO.details/inheritance from Oliver Elphick is a
|
||||
good direction for implementing with exception on more rectrictive child FK
|
||||
constraint (p.3 of message).
|
||||
|
||||
As for me, I was pushed to rollback to scheme with no inheritance at all in
|
||||
my project for now. So I'm very interesting in implementing of right
|
||||
inheritance and I wanted to ask similar question in one of the lists in
|
||||
near
|
||||
future.
|
||||
|
||||
Regards,
|
||||
Dmitry
|
||||
|
||||
|
||||
|
||||
|
||||
---------------------------(end of broadcast)---------------------------
|
||||
TIP 6: Have you searched our list archives?
|
||||
|
||||
http://www.postgresql.org/search.mpl
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue