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postgresql/doc/TODO.detail/inherit
Bruce Momjian a85b67d05b Update TODO list.
2000-01-28 03:46:06 +00:00

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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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Reply-To: "Jonathan davis" <haj@idianet.net>
From: "Jonathan davis" <haj@idianet.net>
To: "Bruce Momjian" <maillist@candle.pha.pa.us>
Cc: "Pgsql-General@Postgresql. Org" <pgsql-general@postgreSQL.org>
Subject: Re: [GENERAL] just little BUG
Date: Fri, 9 Jul 1999 09:46:42 +0200
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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
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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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