From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: Justin Pryzby <pryzby@telsasoft.com>, Robert Haas <robertmhaas@gmail.com>,
        "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: ALTER TABLE .. ALTER COLUMN .. ERROR: attribute .. has wrong type
In-reply-to: <20170106213346.vro3rrghuzabnxmv@alvherre.pgsql>
References: <20170106213346.vro3rrghuzabnxmv@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
	message dated "Fri, 06 Jan 2017 18:33:46 -0300"
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Date: Fri, 06 Jan 2017 17:12:34 -0500
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Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> Tom Lane wrote:
>> Hmm.  The bespoke code for constructing the attno map bothers me;
>> surely there is existing code that does that?  If not, it'd still
>> make more sense to factor it out, I think, because there will be
>> other needs for it in future.

> There isn't any that I could find -- all the existing callers of
> map_variable_attnos build their map in other ways (while walking an
> attribute array at construction time).

[ pokes around... ]  The code I was thinking of is convert_tuples_by_name
in access/common/tupconvert.c.  There's a bit of an API mismatch in that
it wants to wrap the mapping array in a TupleConversionMap struct; but
maybe we could refactor tupconvert.c to offer a way to get just the map
array.

> I also modified the algorithm to use the relcache instead of walking the
> child's attribute list for each parent attribute (that was silly).

Hmm.  That might be better in a big-O sense but I doubt it's faster for
reasonable numbers of columns.

> My rationale when writing the event trigger code was that each command
> would only be published once, for the parent table, not recursively for
> each child.  So only the original expression should be seen.

Oh good; then we're just talking about a localized bug fix and not a
protocol break for event triggers.

			regards, tom lane


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