XPath1 at SQL/XML, count elements returns empty

问题

I am using PostgreSQL 9, that adopts XPath-1 for the SQL/XML standard cumpliance.

This query is ok, returns the expected data:

 SELECT xpath('//img',xhtm) FROM t

but this other one, to count img elements, returns empty (!),

 SELECT xpath('count(//img)',xhtm) FROM t

NOTE 1: a xhtm field content sample,

 <html><p>Hello! <img src="1.png"/></p><img src="2.jpg"/></html>

NOTE 2: of course array_length(xpath('//img',xhtm),1) show the count result, but it is not a XPath counting.

NOTE 3: I don't know if it is a generic SQL/XML peculiarity, or a PosgreSQL Server 9.0.5 specific bug/problem.

回答1:

EDIT: As I noted in the comments below, and as indicated in this SO Answer, this issue was apparently addressed in Postgresql 9.2. That answer states:

With PostgreSQL 9.2 the documentation suddenly has one more sentence covering the xpath function:

If the XPath expression returns a scalar value rather than a node set, a single-element array is returned.

Just what I need! So in relation to the question another valid answer is: Upgrade to PostgreSQL 9.2.

Below is the rest of my original answer:

This seems to be a known limitation in Postgresql's xpath() function - only expressions that evaluate to a node-set return anything; XPath expressions that would return a scalar value just return an empty array.

Some Google searches reveal some discussion about this about 2 years ago primarily from a person named Florian Pflug:

http://postgresql.1045698.n5.nabble.com/PATCH-Bug-in-XPATH-if-expression-returns-a-scalar-value-td4440233.html

He has developed a patch that supposedly fixes the issue and that is attached to the following thread (though I'm not entirely sure this is the latest version):

http://www.postgresql.org/message-id/FC60A4F5-5560-438C-97E8-B6D1FDE1CC2B@phlo.org

There does seem to be some discussion about the pros and cons of his patch, so I'd suggest reading through these threads to be aware of what is involved:

http://www.postgresql.org/message-id/201106291957.04251.rsmogura@softperience.eu http://www.postgresql.org/message-id/201106291934.23089.rsmogura@softperience.eu

回答2:

JLRishe made me aware of your question. I had the same problem in May last year and I discovered that the problem was fixed in PostgreSQL 9.2.

Since version 9.2. the documentation states:

If the XPath expression returns a scalar value rather than a node set, a single-element array is returned.

I just downloaded PostgreSQL 9.2 and ran your example:

postgres=# select version();   
                                                    version                                                    
---------------------------------------------------------------------------------------------------------------
 PostgreSQL 9.2.3 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-52), 64-bit
(1 row)

postgres=# SELECT xpath('count(//img)','<html><p>Hello! <img src="1.png"/></p><img src="2.jpg"/></html>'::xml);
 xpath 
-------
 {2}
(1 row)

来源：https://stackoverflow.com/questions/15309880/xpath1-at-sql-xml-count-elements-returns-empty

标签

postgresql

xpath

count

sqlxml