Tuesday, August 24, 2010

"BY REF" in XML processing

Several XML-related functions that DB2 offers allow to specify the "BY REF" keyword. Is it worth using it? And what does it mean anyway? Let's take a look at it.

DB2 functions like XMLQUERY, XMLEXISTS, and XMLTABLE have a PASSING clause to provide input values, some of the functions also return either a sequence or column values. For all of those values it is possible to specify the option BY REF (see the linked documentation for the functions above). What the BY REF option tells DB2 is to not create copies of the input or output values, but to only move a reference to those values around. This reduces temporary data, uses less of the valuable memory on the database servers, and it is much faster.

SELECT doc FROM myDocs WHERE XMLEXISTS('$d/name' PASSING BY REF doc AS "d" BY REF)

But what happens when the option BY REF is not specified? DB2 takes the default which in all places is BY REF. So sit back, relax, and enjoy a resource-conscious, highly efficient DB2 - by design.

SELECT doc FROM myDocs WHERE XMLEXISTS('$d/name' PASSING doc AS "d")

SELECT doc FROM myDocs WHERE XMLEXISTS('$DOC/name')


BTW: The latter, omitting the PASSING CLAUSE, only works in DB2 for Linux, UNIX, and Windows.

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