From: | Hannu Krosing <hannu(at)skype(dot)net> |
---|---|
To: | Greg Stark <gsstark(at)mit(dot)edu> |
Cc: | Martijn van Oosterhout <kleptog(at)svana(dot)org>, Simon Riggs <simon(at)2ndquadrant(dot)com>, "Jim C(dot) Nasby" <jnasby(at)pervasive(dot)com>, pgsql-hackers(at)postgresql(dot)org, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Subject: | Re: effective SELECT from child tables |
Date: | 2005-10-03 20:32:26 |
Message-ID: | 1128371546.5882.5.camel@fuji.krosing.net |
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Thread: | |
Lists: | pgsql-hackers |
On P, 2005-10-02 at 22:49 -0400, Greg Stark wrote:
> No, I think redefining NULL is a non-starter. This whole thing only has legs
> if Postgres can distinguish between a column that has never been set and a
> column that has NULL.
>
> Actually the only case where I don't see bizarre semantic consequences is the
> case of a newly created column. It would be nice to be able to do:
>
> ALTER TABLE ADD foo integer DEFAULT 1
Probably a different syntax would be better here, perhaps
ALTER TABLE ADD foo integer AS 1 WHEN MISSING;
or somesuch.
> And there's no question of what what the semantics of this are. No question of
> columns magically acquiring a value where they were NULL before.
>
> The *main* reason I would be happy about this is that it would let me add NOT
> NULL columns efficiently. I would expect NOT NULL to be allowed whenever a
> default is provided since there's obviously no way it can cause a problem.
> (I don't follow the "NULL is evil all the time" philosophy but I do try to set
> columns NOT NULL wherever I can. It always annoys me that when adding a new
> column I have to choose between a massive disk intensive batch job or
> compromising the design.)
>
>
> On the other hand if you do
>
> ALTER TABLE ADD foo integer
>
> and then later do
>
> ALTER TABLE ALTER foo SET DEFAULT 1
>
> then there is a window where all those foos are NULL and then they magically
> become 1? That doesn't seem tenable.
Not if you require these two to happen in one transaction to affect
added/missing values.
--
Hannu Krosing <hannu(at)skype(dot)net>
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