Обсуждение: Re: [HACKERS] ALTER TABLE .. ALTER COLUMN .. ERROR: attribute .. has wrong type

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

Tom Lane wrote:
> 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.

Ah, nice gadget.  I think the attached patch should do.

> > 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.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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