Re: generic crosstab ?

From: Andreas <maps(dot)on(at)gmx(dot)net>
To: Samuel Gendler <sgendler(at)ideasculptor(dot)com>
Cc: pgsql-sql(at)postgresql(dot)org
Subject: Re: generic crosstab ?
Date: 2012-04-24 21:34:55
Message-ID: 4F971C7F.7050803@gmx.net
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Am 24.04.2012 22:08, schrieb Samuel Gendler:
>
>
> On Tue, Apr 24, 2012 at 1:01 PM, Andreas <maps(dot)on(at)gmx(dot)net
> <mailto:maps(dot)on(at)gmx(dot)net>> wrote:
>
> Hi,
>
> is there a generic solution to dump the result of a query as a
> crosstab, when I can't know how many columns I will need?
>
> E.g. I get something like this:
>
> id, x
> 1, a
> 1, b
> 1, c
> 2, l
> 2, m
>
>
>
> Yes. You can provide a query which returns the columns to the version
> of the crosstab function which looks like this:
>
> |crosstab(text source_sql, text category_sql)|
>
> It does exactly what you are looking for. The second query returns the
> set of values that act as columns in the final result (the pivot for
> each row in the result returned by the first query). This allows the
> function to correctly insert a null for any column for which there is
> no row in the first query results.
>
>

I got stuck with an error that translates to "Materialisation mode is
needed but is not allowed in this context."
I couldn't figure out what this materialisation mode is, yet.

Could you please have a look at my query sample?
Both queries work for themselves but crosstab() fails. :(
I checked and there are never more than 20 child_ids per parent_id so
there should be enough columns.

select
crosstab (
$$
select
parent_id as row_name,
'x' || row_number() over ( partition by parent_id order by
child_id ) as category,
child_id as value
from
children
order by 1
$$,
$$
select 'x' || generate_series(1, 20) as cat order by 1
$$
);

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