From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: ALTER TABLE .. ALTER COLUMN .. ERROR: attribute .. has wrong type
Date: Mon, 09 Jan 2017 14:10:25 -0500
Message-ID: <26761.1483989025@sss.pgh.pa.us> (raw)
In-Reply-To: <20170109182800.qrkae62kmur3gfeg@alvherre.pgsql>
References: <20170109182800.qrkae62kmur3gfeg@alvherre.pgsql>
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Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> Tom Lane wrote:
>> [ 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.
> Ah, nice gadget. I think the attached patch should do.
Looks reasonable to me.
>>> 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.
> Hm, I was thinking in unreasonable numbers of columns, keeping in mind
> that they can appear in arbitrary order in child tables. Then again,
> that probably seldom occurs in real databases. I suppose this could
> become an issue with table partitioning becoming more common, but I'm
> okay with deferring the optimization work.
It occurred to me that it'd be really easy to improve
convert_tuples_by_name so that, rather than having the inner loop
start from j = 0 every time, it starts from the attribute after the
last match (and loops around if needed, so that it still examines
every child attribute). I think this would keep it at more-or-less
linear time for all but very contrived child tables.
Since your patch is touching that code I won't do anything about it
right now, but maybe later.
regards, tom lane
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Cc: tgl@sss.pgh.pa.us, alvherre@2ndquadrant.com, pryzby@telsasoft.com, robertmhaas@gmail.com
Subject: Re: ALTER TABLE .. ALTER COLUMN .. ERROR: attribute .. has wrong type
In-Reply-To: <26761.1483989025@sss.pgh.pa.us>
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