Re: PostgreSQL vs SQL/XML Standards

ne 30. 12. 2018 v 20:06 odesílatel Chapman Flack <chap(at)anastigmatix(dot)net>
napsal:

> On 12/30/18 03:23, Pavel Stehule wrote:
> > I agree with more stronger detail description about difference between
> > XPath, XPath2 and XQuery.
> >
> > Some like "The XPath 1.0 is ancestor of XPath 2.0, that is part of
> XQuery.
> > ..."
>
> I'll be happy to work on some wording to help detail the differences.
> I don't think it can be condensed to a single sentence, or even if it
> could, it would not serve users well; what's helpful is to know
> the specifics of what changed between XPath 1.0 and the 2.0-3.0-3.1 lineage
> that is now XPath, because that's what helps to understand what the
> challenges will be when porting queries from other systems, how to
> recognize
> when a query won't be possible to port at all, etc.
>
> So I think it needs to be a short section. I am still undecided what's
> the best place in the manual for the section to go:
>
>
+1

> - Added to the introductory material under functions-xml
>
> - A long <footnote> attached to a short remark in that introductory matter
> (is there any precedent? I see there are <footnote>s already in the
> manual, but I haven't checked how long they get)
>
> - A new appendix (or new section in the SQL Conformance appendix)
> linked from a short statement in the functions-xml introductory matter.
>
> > I don't think so link to wiki with any proposals is good idea.
>
> That makes sense. I was thinking only of linking directly to the specific
> section on XPath 1.0 compatibility issues, not to the wiki page as a whole.
> I see in grepping through the *.sgml that there are not many links to the
> wiki.postgresql.org, and the ones that exist are only in release notes,
> Further Information, The Source Code Repository, and sepgsql.
>
> So precedent seems to stand against using a link to the wiki section.
> So, the material /in/ that wiki section [1] (but omitting the two paras
> about the DOCUMENT/CONTENT node-wrapping hack) is more or less what needs
> to go into the new section in the manual.
>
> I assume it's ok to directly include links [2] and [3] out to the w3.org
> compatibility notes ... those are stable URLs to reference material.
>
> === BY REF and BY VALUE ===
>
> How difficult would it be to make the grammar constructs that currently
> accept "BY REF" (only as noise and ignore it) accept and ignore both BY REF
> and BY VALUE? The documentation ought to state that, while both forms are
> accepted and ignored, PostgreSQL's actual behavior corresponds to what the
> standard calls BY VALUE (using a string serialization as the XML datatype's
> internal form makes the BY REF semantics impossible). So it's when people
> try to port queries that explicitly say BY REF that they need to know there
> may be trouble ahead.
>

This is difficult question - a implementation is probably very easy, but it
is hard to accept to possible break compatibility due syntactic sugar.

This is not probably related to just XPath/XQuery question - but it is
related to different design of XML datatype (based on PostgreSQL TOAST)
against ANSI/SQL (Oracle - clob).

So this is complicated topic and my opinion is better to don't touch it
because we can't to fix it, change it - and I am not sure so ANSI/SQL is
significantly better than PostgreSQL implementation.

>
> -Chap
>
>
> >> [1]
> https://wiki.postgresql.org/wiki/PostgreSQL_vs_SQL/XML_Standards#Related_to_version_of_XPath
> >> [2] https://www.w3.org/TR/xpath20/#id-backwards-compatibility
> >> [3]
> https://www.w3.org/TR/2010/REC-xpath-functions-20101214/#xpath1-compatibility
>