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List:       pgsql-performance
Subject:    Re: [PERFORM] Altering a column type - Most efficient way
From:       Tom Lane <tgl () sss ! pgh ! pa ! us>
Date:       2008-07-11 14:27:34
Message-ID: 3462.1215786454 () sss ! pgh ! pa ! us
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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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