From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@commandprompt.com>
Cc: Ow Mun Heng <Ow.Mun.Heng@wdc.com>
Cc: Mario Weilguni <mweilguni@sime.com>
Cc: Pgsql performance <pgsql-performance@postgresql.org>
Subject: Re: Altering a column type - Most efficient way
Date: Fri, 11 Jul 2008 10:27:34 -0400
Message-ID: <3462.1215786454@sss.pgh.pa.us> (raw)
In-Reply-To: <20080711134423.GA4110@alvh.no-ip.org>
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<20080710135756.GD3757@alvh.no-ip.org>
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<20080711134423.GA4110@alvh.no-ip.org>
Alvaro Herrera <alvherre@commandprompt.com> writes:
> Ow Mun Heng wrote:
>> If it were this simple a change, I'm not certain why (I believe) PG is
>> checking each and every row to see if it will fit into the new column
>> definition/type.
> Because the code that does the ALTER TYPE is very generic, and it
> doesn't (yet) have an optimization that tells it to skip the check and
> the possible table rewrite in the cases where it's obviously not needed
> (like this one).
Awhile back I looked into teaching ALTER TYPE that it needn't rewrite
if the type conversion expression parses out as just a Var with
RelabelType, but it seemed that that wouldn't cover very much of the
use-cases where a human thinks that it's "obvious" that no rewrite
is needed. You'd really need to build in hard-wired knowledge about
the behavior of specific coercion functions, which seems entirely
unappealing.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, alvherre@commandprompt.com, Ow.Mun.Heng@wdc.com, mweilguni@sime.com
Subject: Re: Altering a column type - Most efficient way
In-Reply-To: <3462.1215786454@sss.pgh.pa.us>
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