# 22. Virtuoso Functions Guide - xper_cut

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XPATH & XQUERY
Functions Index

## xper_cut

creates a new "persistent XML"document which contains a copy of data pointed by given XPER entity
xper_cut (in source_xperXML_Entity);
Description

As noted in the Storage in Database section, a subtree may be extracted from a document during writing of "persistent XML" entity into field of type LONG VARCHAR. The procedure of converting a subtree into complete document is known as "cutting". Cutting is performed only for "persistent XML" documents, it has nothing common with serialization of XML entities in form of plain XML text.

Usually it is the job of the Server itself who decides whether a cutting operation should be performed or not, without any specific activity at application level. The CPU time occupied due to cutting is up to 10 times greater than the CPU time of plain copying of LONG VARCHAR, but the amount of disk IO is about the same, so the optimization rules discussed below are important only for time-critical, memory-located database applications.

The Virtuoso Server tries to reduce the number of cuttings to an absolute minimum. First of all, cutting is not performed when a given XML entity refers to the root of the document, or to the only child of the root, because the result of such cutting will be identical to original document. In addition, every document remembers the result of last cutting performed on data from this document, so if data of some XML entity are saved in many places without saving of other XML entities between them, cutting will be done only once and plain copying will be done for every subsequent saving.

The only situation when cutting may be seriously optimized by the application developer is in code like the following:

    declare _blank_forms, _plain, _isdn any;
    _blank_forms := xml_persistent ('file://blank_forms.xml');
    _plain = xpath_eval ('/forms/form[@name = ''Phone Installation'']', _blank_forms);
    _isdn = xpath_eval ('/forms/form[@name = ''ISDN Installation'']', _blank_forms);
    for select ID as _id, SERVICE as _service, ADDRESS as _address from CLIENTS do
    {
      if (is_isdn(_service))
        {
          insert into JOB_DETAILS (ID, QUERY_XML) values (_id, _isdn);
        }
      else
        {
          insert into JOB_DETAILS (ID, QUERY_XML) values (_id, _plain);
        }
    }

Calls of xpath_eval are outside the loop, so it is faster than retrieval of suitable form for every selected record. But values of both _plain and _isdn shares the same underlaying XML document and they will be assigned many times by the 'insert' operation. The XML document has no place to cache two results of cuttings, so new cutting will be done every time when _isdn entity is saved after _plain or _plain saved after _isdn. To optimize, it is better to cut them once outside the loop:

    declare _blank_forms, _plain, _isdn any;
    _blank_forms := xml_persistent ('file://blank_forms.xml');
    _plain = xper_cut (xpath_eval ('/forms/form[@name = ''Phone Installation'']', _blank_forms));
    _isdn = xper_cut (xpath_eval ('/forms/form[@name = ''ISDN Installation'']', _blank_forms));
    for select ID as _id, SERVICE as _service from CLIENTS do
    {
      if (is_isdn(_service))
        {
    -- _isdn entity points to the root of its own document, cutting is not needed for root.
          insert into JOB_DETAILS (ID, QUERY_XML) values (_id, _isdn);
        }
      else
        {
    -- similarly, _plain entity points to the root of its own document.
          insert into JOB_DETAILS (ID, QUERY_XML) values (_id, _plain);
        }
    }
    -- If no records found by the 'select' and no inserts done,
    -- then we've made two cuts for nothing...

The current node of the resulting entity is the node that is a copy of the current node of the source entity. In common, the top-level node of the copied subtree becomes the current node of the result. There are two special cases, however. If the source entity is an attribute entity, then the result is also an attribute entity and the attribute name remains the same. If the source entity points to the root of the document, the resulting entity also points to the root, not to its top-level node.

Parameters
source_xper – XML Entity
See Also

xml_doc

xper_doc

xper_right_sibling(), xper_left_sibling(), xper_parent(), xper_root_entity(), xper_tell(), xper_length()