Re: patch: function xmltable

From: Craig Ringer <craig(at)2ndquadrant(dot)com>
To: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: patch: function xmltable
Date: 2016-09-12 04:28:44
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> I'll take a closer read-through shortly.

Missing file. You omitted executor/tableexpr.h from the patch, so I
can't compile.

I've expanded and copy-edited the docs. Some of it is guesswork based
on the references you sent and a glance at the code. Please check my
changes carefully. I found a few surprises, like the fact that DEFAULT
isn't a normal literal, it's an xpath expression evaluated at the same
time as the rowexpression.

Updated patch attached as XMLTABLE-v3 includes the docs changes. Note
that it's missing tableexpr.h. For convenient review or to apply to
your working tree I also attach a diff of just my docs changes as


- Can you send the sample data used to generate the example output?
I'd like to include at least a cut down part of it in the docs to make
it clear how the input correlates with output, and preferably put the
whole thing in an appendix.

- How does it decide what subset of the document to iterate over?
That's presumably rowexpr, which is xpath in postgresql? (I added this
to docs).

- xmlnamespaces clause in docs needs an example for a non-default namespace.

- What effect does xmlnamespaces clause have? Does supplying it allow
you to reference qualified names in xpath? What happens if you don't
specify it for a document that has namespaces or don't define all the
namespaces? What if you reference an undefined namespace in xpath?
What about if an undefined namespace isn't referenced by xpath, but is
inside a node selected by an xpath expression?

- What are the rules for converting the matched XML node into a value?
If the matched node is not a simple text node or lacks a text node as
its single child, what happens?

- What happens if the matched has multiple text node children? This
can happen if, for example, you have something like

some text <!-- comment splits up text node --> other text

- Is there a way to get an attribute as a value? If so, an example
should show this because it's going to be a common need. Presumably
you want node/@attrname ?

- What happens if COLUMNS is not specified at all? It looks like it
returns a single column result set with the matched entries as 'xml'
type, so added to docs, please verify.

- PASSING clause isn't really defined. You can specify one PASSING
entry as a literal/colref/expression, and it's the argument xml
document, right? The external docs you referred to say that PASSING
may have a BY VALUE keyword, alias its argument with AS, and may have
expressions, e.g.

PASSING BY VALUE '<x/>' AS a, '<y/>' AS b

Neither aliases nor multiple entries are supported by the code or
grammar. Should this be documented as a restriction? Do you know if
that's an extension by the other implementation or if it's SQL/XML
standard? (I've drafted a docs update to cover this in the updated

- What does BY REF mean? Should this just be mentioned with a "see
xmlexists(...)" since it seems to be compatibility noise? Is there a
corresponding BY VALUE or similar?

- The parser definitions re-use xmlexists_argument . I don't mind
that, but it's worth noting here in case others do.

- Why do the expression arguments take c_expr (anything allowed in
a_expr or b_expr), not b_expr (restricted expression) ?

- Column definitions are underdocumented. The grammar says they can be
NOT NULL, for example, but I don't see that in any of the references
you mailed me nor in the docs. What behaviour is expected for a NOT
NULL column? I've documented my best guess (not checked closely
against the code, please verify).


Test suggestions:

- Coverage of multiple text() node children of an element, where split
up by comment or similar

- Coverage of xpath that matches a node with child element nodes

More to come. Please review my docs changes in the mean time. I'm
spending a lot more time on this than I expected so I might have to
get onto other things for a while too.

Craig Ringer
PostgreSQL Development, 24x7 Support, Training & Services

Attachment Content-Type Size
proposed-docs-changes.patch text/x-patch 6.3 KB
0001-XMLTABLE-v3.patch.gz application/x-gzip 22.6 KB

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