PostgreSQL absolute over relative xpath location - postgresql

Question

Consider the following xml document that is stored in a PostgreSQL field:
<E_sProcedure xmlns="http://www.minushabens.com/2008/FMSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" modelCodeScheme="Emo_ex" modelCodeSchemeVersion="01" modelCodeValue="EMO_E_PROCEDURA" modelCodeMeaning="Section" sectionID="11">
<tCatSnVsn_Pmax modelCodeScheme="Emodinamica_referto" modelCodeSchemeVersion="01" modelCodeValue="tCat4" modelCodeMeaning="My text"><![CDATA[1]]></tCatSnVsn_Pmax>
</E_sProcedure>
If I run the following query I get the correct result for Line 1, while Line 2 returns nothing:
SELECT
--Line 1
TRIM(BOTH FROM array_to_string((xpath('//child::*[#modelCodeValue="tCat4"]/text()', t.xml_element)),'')) as tCatSnVsn_Pmax_MEANING
--Line2
,TRIM(BOTH FROM array_to_string((xpath('/tCatSnVsn_Pmax/text()', t.xml_element)),'')) as tCatSnVsn_Pmax
FROM (
SELECT unnest(xpath('//x:E_sProcedure', s.XMLDATA::xml, ARRAY[ARRAY['x', 'http://www.minushabens.com/2008/FMSchema']])) AS xml_element
FROM sr_data as s)t;
What's wrong in the xpath of Line 2?

Answer 1

Your second xpath() doesn't return anything because of two problems. First: you need to use //tCatSnVsn_Pmax as the xml_element still starts with <E_sProcedure>. The path /tCatSnVsn_Pmax tries to select a top-level element with that name.
But even then, the second one won't return anything because of the namespace. You need to pass the same namespace definition to the xpath(), so you need something like this:
SELECT (xpath('/x:tCatSnVsn_Pmax/text()', t.xml_element, ARRAY[ARRAY['x', 'http://www.minushabens.com/2008/FMSchema']]))[1] as tCatSnVsn_Pmax
FROM (
SELECT unnest(xpath('//x:E_sProcedure', s.XMLDATA::xml, ARRAY[ARRAY['x', 'http://www.minushabens.com/2008/FMSchema']])) AS xml_element
FROM sr_data as s
)t;
With modern Postgres versions (>= 10) I prefer using xmltable() for anything nontrivial. It makes passing namespaces easier and accessing multiple attributes or elements.
SELECT xt.*
FROM sr_data
cross join
xmltable(xmlnamespaces ('http://www.minushabens.com/2008/FMSchema' as x),
'/x:E_sProcedure'
passing (xmldata::xml)
columns
sectionid text path '#sectionID',
pmax text path 'x:tCatSnVsn_Pmax',
model_code_value text path 'x:tCatSnVsn_Pmax/#modelCodeValue') as xt
For your sample XML, the above returns:
sectionid | pmax | model_code_value
----------+------+-----------------
11 | 1 | tCat4