| From: | Andreas Seltenreich <seltenreich(at)gmx(dot)de> |
|---|---|
| To: | Joachim Zobel <jzobel(at)heute-morgen(dot)de> |
| Cc: | Bruno Wolff III <bruno(at)wolff(dot)to>, pgsql-general(at)postgresql(dot)org |
| Subject: | Re: Limits of SQL |
| Date: | 2005-06-05 21:35:05 |
| Message-ID: | 87ll5osd12.fsf@gate450.dyndns.org |
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| Lists: | pgsql-general |
Joachim Zobel schrob:
> Am Samstag, den 04.06.2005, 15:22 -0500 schrieb Bruno Wolff III:
>> On Sat, Jun 04, 2005 at 21:53:24 +0200,
>> Joachim Zobel <jzobel(at)heute-morgen(dot)de> wrote:
>> > So WITH will allow recursion so I can walk the graph, right? Does this
>> > mean I can recursively join until a terminating condition is reached?
>>
>> It can be used to compute transitive closures, which I think is what
>> you are really looking for.
>
> There aren't any plans to implement grouping by a user defined
> equivalence relation? This is the other thing I am missing. But OK, WITH
Isn't this already possible by representing the relation through its
canonical mapping (i.e. f(a)=f(b) <=> a relates to b)? You could then
use GROUP BY f(x) to group data into its equivalence classes.
regards,
Andreas
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