Add inheritance.

doc/TODO.detail/inheritance

@@ -520,7 +520,7 @@ From pgsql-general-owner+M2136@hub.org Sat Jun  3 23:31:02 2000
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@@ -764,3 +764,348 @@ GPG: 1024D/3E1D0C1C: CA12 09E0 E8D5 8870 5839  932A 614D 4C34 3E1D 0C1C
 
 
 
+From pgsql-hackers-owner+M9621@postgresql.org Mon Jun  4 21:53:36 2001
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+From: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
+To: "Hackers" <pgsql-hackers@postgresql.org>
+Subject: [HACKERS] Question about inheritance
+Date: Tue, 5 Jun 2001 09:42:38 +0800
+Message-ID: <ECEHIKNFIMMECLEBJFIGEENPCAAA.chriskl@familyhealth.com.au>
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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?
+
+Chris
+
+
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+From pgsql-hackers-owner+M9623@postgresql.org Mon Jun  4 22:17:50 2001
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+To: "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
+cc: "Hackers" <pgsql-hackers@postgresql.org>
+Subject: Re: [HACKERS] Question about inheritance 
+In-Reply-To: <ECEHIKNFIMMECLEBJFIGEENPCAAA.chriskl@familyhealth.com.au> 
+References: <ECEHIKNFIMMECLEBJFIGEENPCAAA.chriskl@familyhealth.com.au>
+Comments: In-reply-to "Christopher Kings-Lynne" <chriskl@familyhealth.com.au>
+	message dated "Tue, 05 Jun 2001 09:42:38 +0800"
+Date: Mon, 04 Jun 2001 22:07:42 -0400
+Message-ID: <11249.991706862@sss.pgh.pa.us>
+From: Tom Lane <tgl@sss.pgh.pa.us>
+Precedence: bulk
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+Status: OR
+
+"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.
+
+			regards, tom lane
+
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+Date: Tue, 5 Jun 2001 13:08:58 +1000
+Subject: Re: [HACKERS] Question about inheritance
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+
+
+
+
+>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?
+
+You would either have to check each index in the hierarchy or else have
+a single index across the whole hierarchy and check that. Obviously the
+latter would be generally more useful.
+
+As with all things inheritance, it is usually the right thing, and a good
+default that things be inherited. So ideally, indexes should work across
+whole hierarchies as well as primary, unique and foreign constraints.
+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)
+From: Stephan Szabo <sszabo@megazone23.bigpanda.com>
+To: Christopher Kings-Lynne <chriskl@familyhealth.com.au>
+cc: Hackers <pgsql-hackers@postgresql.org>
+Subject: Re: [HACKERS] Question about inheritance
+In-Reply-To: <ECEHIKNFIMMECLEBJFIGEENPCAAA.chriskl@familyhealth.com.au>
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+
+On Tue, 5 Jun 2001, Christopher Kings-Lynne wrote:
+
+> 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
+allow inherited constraints to be dropped.  "Why does 'select * from
+base;' give me rows where value<10 since there's a check value>=10 
+on the table?"
+
+As Tom said, the unique constraint thing is still questionable which is
+the more meaningful semantics.  If we ever want to allow foreign key
+constraints to inheritance trees, we need *some* way to guarantees
+uniqueness across the tree even if that isn't through the unique
+constraint.
+
+
+---------------------------(end of broadcast)---------------------------
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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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+From: "Dmitry G. Mastrukov" <dmitry@taurussoft.org>
+To: <pgsql-hackers@postgresql.org>
+Subject: Re: [HACKERS] Question about inheritance 
+Date: Tue, 5 Jun 2001 14:17:33 +0400
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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
+
+
+
+
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