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postgresql/doc/TODO.detail/atttypmod
Bruce Momjian 7048c8c6d5 Add mention of CREATE TABLE and atttypmod problems.
2001-12-05 21:07:23 +00:00

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From tgl@sss.pgh.pa.us Wed Nov 21 22:51:02 2001
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Peter Eisentraut <peter_e@gmx.net>,
PostgreSQL Development <pgsql-hackers@postgresql.org>,
stiening@cannon.astro.umass.edu, pgsql-bugs@postgresql.org
Subject: Re: [BUGS] Bug #513: union all changes char(3) column definition
In-Reply-To: <200111220310.fAM3A2V08766@candle.pha.pa.us>
References: <200111220310.fAM3A2V08766@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
message dated "Wed, 21 Nov 2001 22:10:02 -0500"
Date: Wed, 21 Nov 2001 22:51:04 -0500
Message-ID: <27975.1006401064@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ORr
Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Added to TODO:
> * CREATE TABLE AS can not determine column lengths from expressions
> Seems it should be documented. Do we throw an error in these cases?
No. What we do right now is to generate non-length-constrained column
types for the created table.
Your TODO item is too pessimistic: we *do* determine the column length
in simple cases. For example:
regression=# create table foo (f1 char(3));
CREATE
regression=# create table bar as select * from foo;
SELECT
regression=# \d bar
Table "bar"
Column | Type | Modifiers
--------+--------------+-----------
f1 | character(3) |
However, in more complex cases we don't know the column length:
regression=# create table baz as select f1 || 'z' as f1 from foo;
SELECT
regression=# \d baz
Table "baz"
Column | Type | Modifiers
--------+--------+-----------
f1 | bpchar |
The argument here is about how much intelligence it's reasonable to
expect the system to have. It's very clearly not feasible to derive
a length limit automagically in every case. How hard should we try?
regards, tom lane
From pgsql-bugs-owner+M2695=candle.pha.pa.us=pgman@postgresql.org Wed Nov 21 23:16:19 2001
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Wed, 21 Nov 2001 23:08:37 -0500 (EST)
To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Peter Eisentraut <peter_e@gmx.net>,
PostgreSQL Development <pgsql-hackers@postgresql.org>,
stiening@cannon.astro.umass.edu, pgsql-bugs@postgresql.org
Subject: Re: [BUGS] Bug #513: union all changes char(3) column definition
In-Reply-To: <200111220353.fAM3rRg12994@candle.pha.pa.us>
References: <200111220353.fAM3rRg12994@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
message dated "Wed, 21 Nov 2001 22:53:27 -0500"
Date: Wed, 21 Nov 2001 23:08:37 -0500
Message-ID: <28079.1006402117@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-bugs-owner@postgresql.org
Status: OR
Bruce Momjian <pgman@candle.pha.pa.us> writes:
> However, I don't think creating a bpchar
> with no length is a proper solution. Should we just punt to text in
> these cases?
How many special cases like that do you want to put into the allegedly
datatype-independent CREATE TABLE code?
If I thought this were the only case then I'd not object ... but it
looks like a slippery slope from here.
And --- it's not like replacing "bpchar" with "text" actually buys us
any useful new functionality. AFAICS it's just a cosmetic thing.
regards, tom lane
PS: On the other hand, we might consider attacking the problem from
the reverse direction, ie *removing* code. For example, if there
weren't redundant || operators for char and varchar, then every ||
operation would yield text, and the example we're looking at would
work the way you want for free. I've thought for awhile that we
could use a pass through pg_proc and pg_operator to remove some
entries we don't really need.
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From tgl@sss.pgh.pa.us Wed Nov 21 23:08:36 2001
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Peter Eisentraut <peter_e@gmx.net>,
PostgreSQL Development <pgsql-hackers@postgresql.org>,
stiening@cannon.astro.umass.edu, pgsql-bugs@postgresql.org
Subject: Re: [BUGS] Bug #513: union all changes char(3) column definition
In-Reply-To: <200111220353.fAM3rRg12994@candle.pha.pa.us>
References: <200111220353.fAM3rRg12994@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
message dated "Wed, 21 Nov 2001 22:53:27 -0500"
Date: Wed, 21 Nov 2001 23:08:37 -0500
Message-ID: <28079.1006402117@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ORr
Bruce Momjian <pgman@candle.pha.pa.us> writes:
> However, I don't think creating a bpchar
> with no length is a proper solution. Should we just punt to text in
> these cases?
How many special cases like that do you want to put into the allegedly
datatype-independent CREATE TABLE code?
If I thought this were the only case then I'd not object ... but it
looks like a slippery slope from here.
And --- it's not like replacing "bpchar" with "text" actually buys us
any useful new functionality. AFAICS it's just a cosmetic thing.
regards, tom lane
PS: On the other hand, we might consider attacking the problem from
the reverse direction, ie *removing* code. For example, if there
weren't redundant || operators for char and varchar, then every ||
operation would yield text, and the example we're looking at would
work the way you want for free. I've thought for awhile that we
could use a pass through pg_proc and pg_operator to remove some
entries we don't really need.
From pgsql-bugs-owner+M2696=candle.pha.pa.us=pgman@postgresql.org Wed Nov 21 23:26:07 2001
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200111220413.fAM4DSH15042@candle.pha.pa.us>
Subject: Re: [BUGS] Bug #513: union all changes char(3) column definition
In-Reply-To: <28079.1006402117@sss.pgh.pa.us> "from Tom Lane at Nov 21, 2001
11:08:37 pm"
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 21 Nov 2001 23:13:28 -0500 (EST)
cc: Peter Eisentraut <peter_e@gmx.net>,
PostgreSQL Development <pgsql-hackers@postgresql.org>,
stiening@cannon.astro.umass.edu, pgsql-bugs@postgresql.org
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Status: OR
> How many special cases like that do you want to put into the allegedly
> datatype-independent CREATE TABLE code?
>
> If I thought this were the only case then I'd not object ... but it
> looks like a slippery slope from here.
>
> And --- it's not like replacing "bpchar" with "text" actually buys us
> any useful new functionality. AFAICS it's just a cosmetic thing.
>
> regards, tom lane
>
> PS: On the other hand, we might consider attacking the problem from
> the reverse direction, ie *removing* code. For example, if there
> weren't redundant || operators for char and varchar, then every ||
> operation would yield text, and the example we're looking at would
> work the way you want for free. I've thought for awhile that we
> could use a pass through pg_proc and pg_operator to remove some
> entries we don't really need.
Can we convert bpchar to text in create table if no typmod is supplied?
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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From peter_e@gmx.net Thu Nov 22 12:14:01 2001
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Date: Thu, 22 Nov 2001 18:21:17 +0100 (CET)
From: Peter Eisentraut <peter_e@gmx.net>
X-Sender: <peter@peter.localdomain>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: Bruce Momjian <pgman@candle.pha.pa.us>,
PostgreSQL Development <pgsql-hackers@postgresql.org>
Subject: Re: [BUGS] Bug #513: union all changes char(3) column definition
In-Reply-To: <27975.1006401064@sss.pgh.pa.us>
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MIME-Version: 1.0
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Status: OR
Tom Lane writes:
> The argument here is about how much intelligence it's reasonable to
> expect the system to have. It's very clearly not feasible to derive
> a length limit automagically in every case. How hard should we try?
I would like to know what Proprietary database #1 does with
CREATE TABLE one ( a bit(6) );
INSERT INTO one VALUES ( b'101101' );
CREATE TABLE two ( b bit(4) );
INSERT INTO two VALUES ( b'0110' );
CREATE TABLE three AS SELECT a FROM one UNION SELECT b FROM two;
According to SQL92, clause 9.3, the result type of the union is bit(6).
However, it's not possible to store a bit(4) value into a bit(6) field.
Our current solution, "bit(<nothing>)" is even worse because it has no
real semantics at all (but you can store bit(<anything>) in it,
interestingly).
--
Peter Eisentraut peter_e@gmx.net
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