Make xpath() do something useful with XPath expressions that return scalars.

Previously, xpath() simply returned an empty array if the expression did
not yield a node set.  This is useless for expressions that return scalars,
such as one with name() at the top level.  Arrange to return the scalar
value as a single-element xml array, instead.  (String values will be
suitably escaped.)

This change will also cause xpath_exists() to return true, not false,
for such expressions.

Florian Pflug, reviewed by Radoslaw Smogura

doc/src/sgml/func.sgml

@@ -9275,6 +9275,8 @@ SELECT xml_is_well_formed_document('<pg:foo xmlns:pg="http://postgresql.org/stuf
     against the XML value
     <replaceable>xml</replaceable>.  It returns an array of XML values
     corresponding to the node set produced by the XPath expression.
+    If the XPath expression returns a scalar value rather than a node set,
+    a single-element array is returned.
    </para>
 
   <para>

src/backend/utils/adt/xml.c

@@ -126,6 +126,8 @@ static bool print_xml_decl(StringInfo buf, const xmlChar *version,
 static xmlDocPtr xml_parse(text *data, XmlOptionType xmloption_arg,
 		  bool preserve_whitespace, int encoding);
 static text *xml_xmlnodetoxmltype(xmlNodePtr cur);
+static int	xml_xpathobjtoxmlarray(xmlXPathObjectPtr xpathobj,
+								   ArrayBuildState **astate);
 #endif   /* USE_LIBXML */
 
 static StringInfo query_to_xml_internal(const char *query, char *tablename,
@@ -3503,6 +3505,7 @@ SPI_sql_row_to_xmlelement(int rownum, StringInfo result, char *tablename,
  */
 
 #ifdef USE_LIBXML
+
 /*
  * Convert XML node to text (dump subtree in case of element,
  * return value otherwise)
@@ -3554,20 +3557,100 @@ xml_xmlnodetoxmltype(xmlNodePtr cur)
 
 	return result;
 }
-#endif
+
+/*
+ * Convert an XML XPath object (the result of evaluating an XPath expression)
+ * to an array of xml values, which is returned at *astate.  The function
+ * result value is the number of elements in the array.
+ *
+ * If "astate" is NULL then we don't generate the array value, but we still
+ * return the number of elements it would have had.
+ *
+ * Nodesets are converted to an array containing the nodes' textual
+ * representations.  Primitive values (float, double, string) are converted
+ * to a single-element array containing the value's string representation.
+ */
+static int
+xml_xpathobjtoxmlarray(xmlXPathObjectPtr xpathobj,
+					   ArrayBuildState **astate)
+{
+	int			result = 0;
+	Datum		datum;
+	Oid			datumtype;
+	char	   *result_str;
+
+	if (astate != NULL)
+		*astate = NULL;
+
+	switch (xpathobj->type)
+	{
+		case XPATH_NODESET:
+			if (xpathobj->nodesetval != NULL)
+			{
+				result = xpathobj->nodesetval->nodeNr;
+				if (astate != NULL)
+				{
+					int		i;
+
+					for (i = 0; i < result; i++)
+					{
+						datum = PointerGetDatum(xml_xmlnodetoxmltype(xpathobj->nodesetval->nodeTab[i]));
+						*astate = accumArrayResult(*astate, datum,
+												   false, XMLOID,
+												   CurrentMemoryContext);
+					}
+				}
+			}
+			return result;
+
+		case XPATH_BOOLEAN:
+			if (astate == NULL)
+				return 1;
+			datum = BoolGetDatum(xpathobj->boolval);
+			datumtype = BOOLOID;
+			break;
+
+		case XPATH_NUMBER:
+			if (astate == NULL)
+				return 1;
+			datum = Float8GetDatum(xpathobj->floatval);
+			datumtype = FLOAT8OID;
+			break;
+
+		case XPATH_STRING:
+			if (astate == NULL)
+				return 1;
+			datum = CStringGetDatum((char *) xpathobj->stringval);
+			datumtype = CSTRINGOID;
+			break;
+
+		default:
+			elog(ERROR, "xpath expression result type %d is unsupported",
+				 xpathobj->type);
+			return 0;			/* keep compiler quiet */
+	}
+
+	/* Common code for scalar-value cases */
+	result_str = map_sql_value_to_xml_value(datum, datumtype, true);
+	datum = PointerGetDatum(cstring_to_xmltype(result_str));
+	*astate = accumArrayResult(*astate, datum,
+							   false, XMLOID,
+							   CurrentMemoryContext);
+	return 1;
+}
 
 
 /*
  * Common code for xpath() and xmlexists()
  *
  * Evaluate XPath expression and return number of nodes in res_items
- * and array of XML values in astate.
+ * and array of XML values in astate.  Either of those pointers can be
+ * NULL if the corresponding result isn't wanted.
  *
  * It is up to the user to ensure that the XML passed is in fact
  * an XML document - XPath doesn't work easily on fragments without
  * a context node being known.
  */
-#ifdef USE_LIBXML
 static void
 xpath_internal(text *xpath_expr_text, xmltype *data, ArrayType *namespaces,
 			   int *res_nitems, ArrayBuildState **astate)
@@ -3711,26 +3794,13 @@ xpath_internal(text *xpath_expr_text, xmltype *data, ArrayType *namespaces,
 			xml_ereport(xmlerrcxt, ERROR, ERRCODE_INTERNAL_ERROR,
 						"could not create XPath object");
 
-		/* return empty array in cases when nothing is found */
-		if (xpathobj->nodesetval == NULL)
-			*res_nitems = 0;
+		/*
+		 * Extract the results as requested.
+		 */
+		if (res_nitems != NULL)
+			*res_nitems = xml_xpathobjtoxmlarray(xpathobj, astate);
 		else
-			*res_nitems = xpathobj->nodesetval->nodeNr;
-
-		if (*res_nitems && astate)
-		{
-			*astate = NULL;
-			for (i = 0; i < xpathobj->nodesetval->nodeNr; i++)
-			{
-				Datum		elem;
-				bool		elemisnull = false;
-
-				elem = PointerGetDatum(xml_xmlnodetoxmltype(xpathobj->nodesetval->nodeTab[i]));
-				*astate = accumArrayResult(*astate, elem,
-										   elemisnull, XMLOID,
-										   CurrentMemoryContext);
-			}
-		}
+			(void) xml_xpathobjtoxmlarray(xpathobj, astate);
 	}
 	PG_CATCH();
 	{

src/test/regress/expected/xml.out

@@ -601,6 +601,42 @@ SELECT xpath('//@value', '<root value="&lt;"/>');
  {&lt;}
 (1 row)
 
+SELECT xpath('''<<invalid>>''', '<root/>');
+           xpath           
+---------------------------
+ {&lt;&lt;invalid&gt;&gt;}
+(1 row)
+
+SELECT xpath('count(//*)', '<root><sub/><sub/></root>');
+ xpath 
+-------
+ {3}
+(1 row)
+
+SELECT xpath('count(//*)=0', '<root><sub/><sub/></root>');
+  xpath  
+---------
+ {false}
+(1 row)
+
+SELECT xpath('count(//*)=3', '<root><sub/><sub/></root>');
+ xpath  
+--------
+ {true}
+(1 row)
+
+SELECT xpath('name(/*)', '<root><sub/><sub/></root>');
+ xpath  
+--------
+ {root}
+(1 row)
+
+SELECT xpath('/nosuchtag', '<root/>');
+ xpath 
+-------
+ {}
+(1 row)
+
 -- Test xmlexists and xpath_exists
 SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY REF '<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>');
  xmlexists 
@@ -614,6 +650,12 @@ SELECT xmlexists('//town[text() = ''Cwmbran'']' PASSING BY REF '<towns><town>Bid
  t
 (1 row)
 
+SELECT xmlexists('count(/nosuchtag)' PASSING BY REF '<root/>');
+ xmlexists 
+-----------
+ t
+(1 row)
+
 SELECT xpath_exists('//town[text() = ''Toronto'']','<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>'::xml);
  xpath_exists 
 --------------
@@ -626,6 +668,12 @@ SELECT xpath_exists('//town[text() = ''Cwmbran'']','<towns><town>Bidford-on-Avon
  t
 (1 row)
 
+SELECT xpath_exists('count(/nosuchtag)', '<root/>'::xml);
+ xpath_exists 
+--------------
+ t
+(1 row)
+
 INSERT INTO xmltest VALUES (4, '<menu><beers><name>Budvar</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
 INSERT INTO xmltest VALUES (5, '<menu><beers><name>Molson</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
 INSERT INTO xmltest VALUES (6, '<myns:menu xmlns:myns="http://myns.com"><myns:beers><myns:name>Budvar</myns:name><myns:cost>free</myns:cost><myns:name>Carling</myns:name><myns:cost>lots</myns:cost></myns:beers></myns:menu>'::xml);

src/test/regress/expected/xml_1.out

@@ -516,6 +516,42 @@ LINE 1: SELECT xpath('//@value', '<root value="&lt;"/>');
                                  ^
 DETAIL:  This functionality requires the server to be built with libxml support.
 HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT xpath('''<<invalid>>''', '<root/>');
+ERROR:  unsupported XML feature
+LINE 1: SELECT xpath('''<<invalid>>''', '<root/>');
+                                        ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT xpath('count(//*)', '<root><sub/><sub/></root>');
+ERROR:  unsupported XML feature
+LINE 1: SELECT xpath('count(//*)', '<root><sub/><sub/></root>');
+                                   ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT xpath('count(//*)=0', '<root><sub/><sub/></root>');
+ERROR:  unsupported XML feature
+LINE 1: SELECT xpath('count(//*)=0', '<root><sub/><sub/></root>');
+                                     ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT xpath('count(//*)=3', '<root><sub/><sub/></root>');
+ERROR:  unsupported XML feature
+LINE 1: SELECT xpath('count(//*)=3', '<root><sub/><sub/></root>');
+                                     ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT xpath('name(/*)', '<root><sub/><sub/></root>');
+ERROR:  unsupported XML feature
+LINE 1: SELECT xpath('name(/*)', '<root><sub/><sub/></root>');
+                                 ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT xpath('/nosuchtag', '<root/>');
+ERROR:  unsupported XML feature
+LINE 1: SELECT xpath('/nosuchtag', '<root/>');
+                                   ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
 -- Test xmlexists and xpath_exists
 SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY REF '<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>');
 ERROR:  unsupported XML feature
@@ -529,6 +565,12 @@ LINE 1: ...sts('//town[text() = ''Cwmbran'']' PASSING BY REF '<towns><t...
                                                              ^
 DETAIL:  This functionality requires the server to be built with libxml support.
 HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT xmlexists('count(/nosuchtag)' PASSING BY REF '<root/>');
+ERROR:  unsupported XML feature
+LINE 1: ...LECT xmlexists('count(/nosuchtag)' PASSING BY REF '<root/>')...
+                                                             ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
 SELECT xpath_exists('//town[text() = ''Toronto'']','<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>'::xml);
 ERROR:  unsupported XML feature
 LINE 1: ...ELECT xpath_exists('//town[text() = ''Toronto'']','<towns><t...
@@ -541,6 +583,12 @@ LINE 1: ...ELECT xpath_exists('//town[text() = ''Cwmbran'']','<towns><t...
                                                              ^
 DETAIL:  This functionality requires the server to be built with libxml support.
 HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT xpath_exists('count(/nosuchtag)', '<root/>'::xml);
+ERROR:  unsupported XML feature
+LINE 1: SELECT xpath_exists('count(/nosuchtag)', '<root/>'::xml);
+                                                 ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
 INSERT INTO xmltest VALUES (4, '<menu><beers><name>Budvar</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
 ERROR:  unsupported XML feature
 LINE 1: INSERT INTO xmltest VALUES (4, '<menu><beers><name>Budvar</n...

src/test/regress/sql/xml.sql

@@ -177,12 +177,20 @@ SELECT xpath('//loc:piece/@id', '<local:data xmlns:local="http://127.0.0.1"><loc
 SELECT xpath('//b', '<a>one <b>two</b> three <b>etc</b></a>');
 SELECT xpath('//text()', '<root>&lt;</root>');
 SELECT xpath('//@value', '<root value="&lt;"/>');
+SELECT xpath('''<<invalid>>''', '<root/>');
+SELECT xpath('count(//*)', '<root><sub/><sub/></root>');
+SELECT xpath('count(//*)=0', '<root><sub/><sub/></root>');
+SELECT xpath('count(//*)=3', '<root><sub/><sub/></root>');
+SELECT xpath('name(/*)', '<root><sub/><sub/></root>');
+SELECT xpath('/nosuchtag', '<root/>');
 
 -- Test xmlexists and xpath_exists
 SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY REF '<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>');
 SELECT xmlexists('//town[text() = ''Cwmbran'']' PASSING BY REF '<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>');
+SELECT xmlexists('count(/nosuchtag)' PASSING BY REF '<root/>');
 SELECT xpath_exists('//town[text() = ''Toronto'']','<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>'::xml);
 SELECT xpath_exists('//town[text() = ''Cwmbran'']','<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>'::xml);
+SELECT xpath_exists('count(/nosuchtag)', '<root/>'::xml);
 
 INSERT INTO xmltest VALUES (4, '<menu><beers><name>Budvar</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);
 INSERT INTO xmltest VALUES (5, '<menu><beers><name>Molson</name><cost>free</cost><name>Carling</name><cost>lots</cost></beers></menu>'::xml);