Fwd: xpath question

>
> Sean Davis wrote:
> > Just a quick question, not being a big xpath or xml user in the past.
> >
> > Why does this:
> >
> > select xpath('//Abstract/AbstractText[1]',content) from medlinexml;
> >
> > return an array for each entry rather than a single xml entry? I
> > thought that appending the [1] should not return an array, but a
> > single xml value instead.
> >
> > Thanks,
> > Sean
> >
> > example output below
> >
> > {"<Abstract>
> > <AbstractText>Strains of Pseudomonas aeruginosa, isolated from the
> > sputum of relatively fit patients with cystic fibrosis (CF) who had
> > been recently colonized by the organism, showed typical cultural and
> > serologic characteristics. The majority of strains of P. aeruginosa
> > isolated from CF patients with chronic bronchopulmonary infection had
> > 3 distinctive features, loss of 0 serotype reaction, expression of a
> > new somatic antigen, and sensitivity to normal human serum. Patients
> > with organisms with one or two of these features were more severely
> > affected by the disease. The appearance of these variants may
> > represent a critical stage in the progression of CF.</AbstractText>
> > </Abstract>"}
> > {"<Abstract>
> > <AbstractText>The changes in airway caliber and plasma cyclic-AMP
> > levels after intravenously administered aminophylline, and the effect
> > of DL- and D-propranolol on these responses have been investigated in
> > a double-blind manner in normal subjects. Aminophylline 5.6 mg/kg was
> > given intravenously over a 10-min period and the airway response was
> > measured as change in specific airway conductance (delta SGaw) in the
> > body plethysmograph. In the initial study in 6 subjects, orally
> > administered placebo or propranolol was followed 2 h later by
> > intravenously administered aminophylline. Neither placebo nor
> > propranolol alone caused any change in SGaw at 2 h. After placebo,
> > intravenously injected aminophylline produced a 30% increase in SGaw,
> > reaching a peak 5 min after injection. This response was equivalent to
> > 77% of the maximal response to 400 micrograms inhaled albuterol in the
> > same subjects. After propranolol, the airway response to aminophylline
> > was attenuated, with a 53% reduction in delta SGaw at the time of peak
> > response. In a further study on 6 subjects, intravenously given
> > aminophylline produced a 25% increase in SGaw and a 51% increase in
> > plasma cyclic-AMP levels after placebo tablets. Pretreatment with 40
> > and 80 mg DL-propranolol caused a dose-dependent reduction of both the
> > airway and plasma cyclic-AMP response to aminophylline. The airway
> > response to aminophylline was not attenuated by D-propranolol so the
> > effect of DL-propranolol is thought to be due to beta-adrenoceptor
> > blockade. The absence of any detectable change in SGaw after
> > DL-propranolol suggests there is little resting sympathetic tone to
> > the airways in normal subjects. In the absence of sympathetic
> > stimulation, the rapid response to aminophylline is unlikely to be due
> > to phosphodiesterase inhibition. The attenuation of the airway and
> > cyclic-AMP response by propranolol suggests that part of the action of
> > aminophylline may be due to beta-agonist activity.</AbstractText>
> > </Abstract>"}
> >
> > ---------------------------(end of broadcast)---------------------------
> > TIP 3: Have you checked our extensive FAQ?
> >
> > http://www.postgresql.org/docs/faq
> >
> >
> hi your xpath is working correct. You have two results as you have to
> parent nodes 'Abstract'
>
> so if you want only first text from all Abstract node, you should give
> one more condition as
>
> /Abstract[1]/AbstractText[1]

Thanks, Pavel for the reply. However, I looked a bit more and it
appears that xpath always returns an xml array in 8.3b2.

annodb=# \df xpath;
List of functions
Schema | Name | Result data type | Argument data types
------------+-------+------------------+---------------------
pg_catalog | xpath | xml[] | text, xml
pg_catalog | xpath | xml[] | text, xml, text[]
(2 rows)

So, there is not a way to force a single element to be returned as far
as I can see. I did an equivalent example to the one you suggested:

annodb=# select xpath('/MedlineCitation/PMID/text()',content) from
medline.medlinexml limit 10;
xpath
------------
{10111733}
{10145466}
{10111734}
{10111735}
{1830312}
{1830313}
{1830314}
{1830315}
{1830316}
{1713217}
(10 rows)

annodb=# select xpath('/MedlineCitation[1]/PMID[1]/text()',content)
from medline.medlinexml limit 10;
xpath
-----------
{1859432}
{1859433}
{1859434}
{1650203}
{1859435}
{1859436}
{1907139}
{1677568}
{1859437}
{1830481}
(10 rows)

One can select the first element of the array like this, if necessary:

annodb=# select
(xpath('/MedlineCitation[1]/PMID[1]/text()',content))[1] from
medline.medlinexml limit 10;
xpath
---------
1678397
1869692
1869697
1869698
1869693
1869694
1869695
1869696
1869699
1869700
(10 rows)

Sean