[GENERAL] xpath() subquery for empty array

[Roy Walter]

In postgres 8.4 When running xpath() queries it seems that empty results 
are always returned. So if I query a table containing 1000 XML documents 
a 1000 rows will always be fetched even if the xpath() element of the 
query only matches 10 documents.

The documentation states:
   The function |xpath| evaluates the XPath expression /xpath/ against
   the XML value /xml/. It returns an array of XML values corresponding
   to the node set produced by the XPath expression.

Yet it seems that empty node sets (arrays) are also returned. So, my 
thought was to run a subquery to eliminate empty arrays, e.g.:

   SELECT x
   FROM (SELECT xpath('//entry[contains(p, ''searchtext'')]/@*', docxml)
   AS x FROM docs) AS y WHERE x <> [test for empty array?????????]

How do I test for an empty array in postgres?

Thanks in advance,
Roy