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From tgl@sss.pgh.pa.us Sun May 23 12:32:34 1999
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Received: from sss.sss.pgh.pa.us (sss.pgh.pa.us [206.210.65.6])
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for <maillist@candle.pha.pa.us>; Sun, 23 May 1999 12:32:33 -0400 (EDT)
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by sss.sss.pgh.pa.us (8.9.1/8.9.1) with ESMTP id MAA19926;
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Sun, 23 May 1999 12:32:01 -0400 (EDT)
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To: Bruce Momjian <maillist@candle.pha.pa.us>
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cc: PostgreSQL-development <pgsql-hackers@postgreSQL.org>
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Subject: Re: [HACKERS] DEFAULT fixed
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In-reply-to: Your message of Sat, 22 May 1999 21:12:19 -0400 (EDT)
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<199905230112.VAA13489@candle.pha.pa.us>
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Date: Sun, 23 May 1999 12:32:01 -0400
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Message-ID: <19923.927477121@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Status: ROr
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Bruce Momjian <maillist@candle.pha.pa.us> writes:
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>> It might be best to just bite the bullet and make the parser carry
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>> around both the type's OID and typmod at all times.
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> I will try to add it, but I must not that there are some cases where I
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> don't have access to the atttypmod of the result, so it may not be
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> possible to do it in every case. Perhaps I should just leave this for
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> post 6.5, because we don't have any other bug reports on it.
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After further thought, I think this may be a more difficult and subtle
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issue than we've realized. In the current state of the system, there
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are many places where you have a value that you can only know the type
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OID for, not atttypmod --- specifically, any intermediate expression
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result. Barring reworking the entire function-call mechanism to pass
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atttypmod around, that's not going to be possible to change.
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The only context where you really know atttypmod is where you have
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just fetched a value out of a table column or are about to store a
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value into a table column. When storing, you need to be prepared to
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coerce the given value to the right type if *either* type OID or
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atttypmod is different --- but, in general, you don't know atttypmod
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for the given value. (In the cases I know of, you can deduce it by
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examining the value itself, but this requires type-specific knowledge.)
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So on the whole I think this is something that has to be dealt with
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at the point of storing data into a tuple. Maybe we need a new
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fundamental operation for types that pay attention to atttypmod:
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"make this value match the typmod of the target column, which is
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thus-and-so". Trying to attack the problem from the source side by
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propagating typmod all around the parser is probably doomed to failure,
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because there will be many contexts where there's no way to know it.
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Since you have a fix for the only symptom reported to date, I'm
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inclined to agree that we should leave well enough alone for now;
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there are other, bigger, problems that we need to address for 6.5.
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But I think we'll have to come back to this issue later.
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regards, tom lane
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