xpath_doublexpath_double xpath_double 介绍 xpath_double(xml, xpath) - 返回一个双精度数值，如果没有找到匹配项，则返回零；如果找到了匹配项但该值是非数值，则返回NaN。 Examples: > SELECT xpath_double('<a><b>1</b><b>2</b></a>', 'sum(a/b)'); 3.0 Since: 2.0.0xpath_booleanxpath_float