path: root/doc/TODO.detail/inheritance

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From: Chris Bitmead <chris.bitmead@bigfoot.com>
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Subject: Re: [HACKERS] ALTER TABLE ADD COLUMN
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Bruce Momjian wrote:

> Our TODO now has:
> 
>         * ALTER TABLE ADD COLUMN to inherited table put column in wrong place
> 
> I don't think any of us understand the issues on this one.

Let me guess at the problem. When you add a column, it doesn't change
all the records, therefore the column must be added at the end. This
means that the columns will not be in the same order as if you had
created them from scratch.

There seem to be three solutions:
a) Go to a much more sophisticated schema system, with versions and
version numbers (fairly hard but desirable to fix other schema change
problems). Then insert the column in the position it is supposed to be
in.

b) Fix the copy command to input and output the columns, not in the
order they are in, but in the order they would be in on re-creation.

c) make the copy command take arguments specifying the field names, like
INSERT can do.

I think it would be good if Postgres had all 3 features. Probably (b) is
the least work.


From owner-pgsql-general@hub.org Fri Jul  9 04:01:16 1999
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To: "Bruce Momjian" <maillist@candle.pha.pa.us>
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Subject: Re: [GENERAL] just little BUG
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>[Charset iso-8859-1 unsupported, filtering to ASCII...]
>> hello all
>> 
>> normaly a UNIQUE PRIMARY KEY is unique but 
>> when you use a heritage, you can insert a duplicate key !!!!
>
>I assume you mean inheritance.
>
>Can you send us a little test sample please? 
>
>-- 
hello all

this is the problem:

example:

test=> CREATE TABLE MAN(name char(10) UNIQUE PRIMARY KEY);T

test=> CREATE TABLE PROFESSOR(scool char(20))INHERITS(MAN);

test=> INSERT INTO PROFESSOR(name) VALUES('DAVIS');
INSERT 54424 1

test=> INSERT INTO PROFESSOR(name) VALUES('DAVIS');
INSERT 54425 1









From owner-pgsql-hackers@hub.org Tue Apr 20 10:34:34 1999
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Does the following indicate a bug? It sure is wierd. Maybe some of these
statements aren't supported by postgresql (??), but the outcome doesn't
make sense to me.

httpd=> CREATE TABLE x (y text);
CREATE
httpd=> CREATE VIEW z AS select * from x;
CREATE
httpd=> CREATE TABLE a (b text) INHERITS(z);
CREATE
httpd=> INSERT INTO x VALUES ('foo');
INSERT 168602 1
httpd=> select * from z*;
y  
---
foo
foo
(2 rows)

How did we suddenly get two rows??

-- 
Chris Bitmead
http://www.bigfoot.com/~chris.bitmead
mailto:chris.bitmead@bigfoot.com


From owner-pgsql-hackers@hub.org Tue May 25 11:01:16 1999
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To: pgsql-hackers@postgreSQL.org
Subject: [HACKERS] INSERT INTO view means what exactly?
Date: Tue, 25 May 1999 10:42:39 -0400
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From: Tom Lane <tgl@sss.pgh.pa.us>
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With current sources:

regression=> CREATE TABLE x (y text);
CREATE
regression=> CREATE VIEW z AS select * from x;
CREATE
regression=> INSERT INTO x VALUES ('foo');
INSERT 411635 1
regression=> INSERT INTO z VALUES ('bar');
INSERT 411636 1
regression=> select * from x;
y
---
foo
(1 row)

regression=> select * from z;
y
---
foo
(1 row)

OK, where'd tuple 411636 go?  Seems to me that the insert should either
have been rejected or caused an insert into x, depending on how
transparent you think views are (I always thought they were
read-only?).  Dropping the data into never-never land and giving a
misleading success response code is not my idea of proper behavior.

			regards, tom lane


From owner-pgsql-hackers@hub.org Mon Jan 24 23:46:25 2000
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To: Don Baccus <dhogaza@pacifier.com>
cc: "Hiroshi Inoue" <Inoue@tpf.co.jp>, "Peter Eisentraut" <peter_e@gmx.net>,
        "PostgreSQL Development" <pgsql-hackers@postgreSQL.org>
Subject: Re: [HACKERS] Happy column dropping 
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Comments: In-reply-to Don Baccus <dhogaza@pacifier.com>
	message dated "Mon, 24 Jan 2000 18:41:37 -0800"
Date: Mon, 24 Jan 2000 22:57:12 -0500
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Don Baccus <dhogaza@pacifier.com> writes:
> Just a reality check for my learning of the internals.  Out of curiousity
> I coincidently have spent the last hour looking to see how add column's
> implemented.  It doesn't appear to do anything other than the new attribute
> to the proper system table.  heap_getattr() just returns null if you ask
> for an attribute past the end of the tuple.  

> This would appear to be (at least one reason) why you can't add a "not null"
> constraint to a column you're adding to an existing relation, or set the
> new column to some non-null default value.

> Correct?  (again, to see if my eyeballs and brain are working in synch
> tonight)

Yup, that's about the size of it.  ADD COLUMN doesn't actually touch the
table itself, so it can only add a column that's initially all NULLs.
And even this depends on some uncomfortable assumptions about the
robustness of heap_getattr().  I have always wondered whether it works
if you ADD COLUMN a 33'rd column (or anything that is just past the
next padding boundary for the null-values bitmap).

Another problem with it is seen when you do a recursive ADD COLUMN in
an inheritance tree.  The added column has the first free column number
in each table, which generally means that it has different numbers in
the children than in the parent.  There are some kluges to make this
sort-of-work for simple cases, but a lot of stuff fails unpleasantly
--- Chris Bitmead can show you some scars from that, IIRC.

> Does your comment imply that it's planned to change this, i.e. actually
> add the new column to each tuple in the relation rather than use the
> existing, somewhat elegant hack?

That's what I would like to see: all the children should have the
same column numbers for all columns that they inherit from the parent.

(Now, this would mean not only physically altering the tuples of
the children, but also renumbering their added columns, which has
implications on stored rules and triggers and so forth.  It'd be
painful, no doubt about it.  Still, I'd rather pay the price in the
seldom-used ADD COLUMN case than try to deal with out-of-sync column
numbers in many other, more commonly exercised, code paths.)

			regards, tom lane

************

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From: Hannu Krosing <hannu@tm.ee>
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To: Don Baccus <dhogaza@pacifier.com>
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        "Ross J. Reedstrom" <reedstrm@wallace.ece.rice.edu>,
        PostgreSQL Development <pgsql-hackers@postgreSQL.org>
Subject: Re: Happy column adding (was RE: [HACKERS] Happy columndropping)
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Don Baccus wrote:
> 
> Ahhh...yes.  I haven't looked at the inheritance code, yet, but I see
> what you're saying.  I think.  Do child-table columns follow parent-table
> columns by some chance (in today's absolute column number scheme)?
> 
> >If we were willing to hardwire the assumption that DROP COLUMN never
> >physically drops a column, but only hides it and adjusts logical column
> >numbers, then the physical column numbers could serve as permanent IDs;
> >so we'd only need two numbers not three.  This might be good, or not.
> 
> Yes.  But if I'm right about how child-table columns are numbered,
> wouldn't add column still cause a problem, i.e. you'd still have to
> change their physical column number?

If we allow deleted column as a basic feature of postgres,
it could be like that 

base:    COL1 | COL2 | COL3 
child:   COL1 | COL2 | COL3 | COL4

after add column 5 to base table

base:    COL1 | COL2 | COL3 | del4 | COL5 
child:   COL1 | COL2 | COL3 | COL4 | COL5

after add column 6 to child

base:    COL1 | COL2 | COL3 | del4 | COL5 
child:   COL1 | COL2 | COL3 | COL4 | COL5 | COL6

after drop column 2 from base table

base:    COL1 | del2 | COL3 | del4 | COL5 
child:   COL1 | del2 | COL3 | COL4 | COL5 | COL6

dropping column from child table that is not a deleted column in 
parent is not allowed.

The delN columns are always NULLed on reading tuple and are written as proper 
null columns (taking up space only in NULL bitmask)

multiple inheritance is tricky and _requires_ unique column ids maybe oids
from pg_attribute to be doable.

-----------------
Hannu

************

From owner-pgsql-hackers@hub.org Thu Jan 27 11:48:26 2000
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From: Peter Eisentraut <peter_e@gmx.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: PostgreSQL Development <pgsql-hackers@postgreSQL.org>
Subject: Re: [HACKERS] Column ADDing issues 
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On 2000-01-25, Tom Lane mentioned:

> > Everything has its order and it's not like the inheritance as such is
> > broken.
> 
> Yes, a whole bunch of stuff is broken after this happens.  Go back and
> consult the archives --- or maybe Chris Bitmead will fill you in; he's
> got plenty of scars to show for this set of problems.  (All I recall
> offhand is that pg_dump and reload can fail to generate a working
> database.)  The bottom line is that it would be a lot nicer if column c
> had the same column position in both the parent table and the child
> table(s).

This should be fixed in pg_dump by infering something via the oids of the
pg_attribute entries. No need to mess up the backend for it.

Maybe pg_dump should optionally dump schemas in terms of insert into
pg_something commands rather than actual DDL. ;)

> 
> I suggest you be very cautious about messing with ALTER TABLE until you
> understand why inheritance makes it such a headache ;-)

I'm just trying to get the defaults and constraints working. If
inheritance stays broken the way it previously was, it's beyond my
powers. But I get the feeling that people rather not alter their tables
unless they have *perfect* alter table commands. I don't feel like arguing
with them, they'll just have to do without then.


-- 
Peter Eisentraut                  Sernanders väg 10:115
peter_e@gmx.net                   75262 Uppsala
http://yi.org/peter-e/            Sweden



************

From pgsql-general-owner+M2136@hub.org Sat Jun  3 23:31:02 2000
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From: Peter Eisentraut <peter_e@gmx.net>
To: ldm@apartia.com
cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] child table doesn't inherit PRIMARY KEY?
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Louis-David Mitterrand writes:

> When creating a child (through CREATE TABLE ... INHERIT (parent)) it
> seems the child gets all of the parent's contraints _except_ its PRIMARY
> KEY. Is this normal?

It's kind of a bug.


-- 
Peter Eisentraut                  Sernanders väg 10:115
peter_e@gmx.net                   75262 Uppsala
http://yi.org/peter-e/            Sweden


From sszabo@megazone23.bigpanda.com Fri Jan 19 12:37:34 2001
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Date: Fri, 19 Jan 2001 09:37:02 -0800 (PST)
From: Stephan Szabo <sszabo@megazone23.bigpanda.com>
To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] child table doesn't inherit PRIMARY KEY?
In-Reply-To: <200101190457.XAA13895@candle.pha.pa.us>
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Probably, since I see it in near recent sources (and it affects
UNIQUE as well.  As I remember it, the last discussion on this couldn't
determine what the correct behavior for unique/primary key constraints
was in the inheritance case (is it a single unique hierarchy through
all the tables [would be needed for fk to inheritance trees] or
separate unique constraints for each table [which would be similar
to how many people seem to currently use postgres inheritance as a 
shortcut]). 

On Thu, 18 Jan 2001, Bruce Momjian wrote:

> Does this bug still exist?
> 
> [ Charset ISO-8859-1 unsupported, converting... ]
> > Louis-David Mitterrand writes:
> > 
> > > When creating a child (through CREATE TABLE ... INHERIT (parent)) it
> > > seems the child gets all of the parent's contraints _except_ its PRIMARY
> > > KEY. Is this normal?


From sszabo@megazone23.bigpanda.com Wed Jan 24 14:26:12 2001
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Date: Wed, 24 Jan 2001 11:25:35 -0800 (PST)
From: Stephan Szabo <sszabo@megazone23.bigpanda.com>
To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: PostgreSQL-development <pgsql-hackers@postgreSQL.org>
Subject: Re: [GENERAL] child table doesn't inherit PRIMARY KEY?
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On Wed, 24 Jan 2001, Bruce Momjian wrote:

> 
> OK, what do people want to do with this item?  Add to TODO list?
> 
> Seems making a separat unique constraint would be easy to do and be of
> value to most users.

The problem is that doing that will pretty much guarantee that we won't
be doing foreign keys to inheritance trees without changing that behavior
and we've seen people asking about adding that too.  I think that this
falls into the general category of "Make inheritance make sense" (Now 
there's a todo item :) )  Seriously, I think the work on how inheritance
is going to work will decide this, maybe we end up with a real inheritance
tree system and something that works like the current stuff in which case
I'd say it's probably one unique for the former and one per for the
latter.




From olly@lfix.co.uk Wed Jan 24 16:41:45 2001
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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> 
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
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
PGP: 1024R/32B8FAA1: 97 EA 1D 47 72 3F 28 47  6B 7E 39 CC 56 E4 C1 47
GPG: 1024D/3E1D0C1C: CA12 09E0 E8D5 8870 5839  932A 614D 4C34 3E1D 0C1C
                 ========================================
     "If anyone has material possessions and sees his
      brother in need but has no pity on him, how can the
      love of God be in him?"
                                    I John 3:17 



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>
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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.

			regards, tom lane

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From pgsql-hackers-owner+M9664@postgresql.org Tue Jun  5 17:56:17 2001
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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.


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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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