Re: [GENERAL] possible to create multivalued index from xpath() results in 8.3?

Am Mittwoch, 21. November 2007 schrieb Tom Lane:
> "Matt Magoffin" <postgresql(dot)org(at)msqr(dot)us> writes:
> > Ugh, you're right of course! Somehow I had this wrong. So I tried to
> > create an index on the xml[] result by casting to text[] but I got the
> > "function must be immutable" error. Is there any reason the xml[] to
> > text[] cast is not immutable?
>
> Hmm ... I see that xmltotext() is marked 'stable' in pg_proc.h,
> but texttoxml() is marked 'immutable', which is at best inconsistent.
> It looks to me like they both depend on the GUC setting "xmloption",
> which would mean they should both be stable. Peter, is there a bug
> there?

Yeah, that doesn't look right.

> Also, is there a way to get rid of the GUC dependency so that
> there's a reasonably safe way to index XML values?

We could drop the dependency in xmltotext() with little loss of functionality.
The spec doesn't allow casts between xml and text (varchar) at all. The way
I appear to have derived the current behavior from the spec is that this is
interpreted as an implicit XMLSERIALIZE call in the context of a prepared
statement, which is defined to observe the XML option, as per clause 17.3
(part 14). This was the clostest piece of spec that described conversion
from xml to character types. Now with the xpath functionality, there is
certainly a strong use case for ignoring this altogether and just serializing
with the XML option set to "content".

--
Peter Eisentraut
http://developer.postgresql.org/~petere/