Extremely pureXML in DB2 10 for z/OS

60 Extremely pureXML in DB2 10 for z/OS

4.3 Multiple version concurrency control for XML

DB2 supports multiple versions of an XML document in an XML column if the base table

space

for the table that contains the XML column is also a universal table space and created

in DB2 10 NFM

. All XML columns in the table support multiple versions.

With XML versions, when you insert an XML document into an XML column, DB2 assigns a

version number to the XML document. If the entire XML document is updated, DB2 creates a

new version of the document in the XML table. If a portion of the XML document is updated,

DB2 creates a new version of the updated portion. When DB2 uses XML versions, more data

set space is required than when versions are not used. However, DB2 periodically deletes

versions that are no longer needed. In addition, you can run the REORG utility against the

XML table space that contains the XML document to remove unneeded versions. DB2

removes versions of a document when update operations that require the versions are

committed, and when no readers reference the unneeded versions.

4.3.1 Example of improved concurrency with XML versions

This example demonstrates how multiple XML versions can improve concurrency when the

same XML documents are modified multiple times within the same transaction.

Suppose that table T1, which is in a universal table space, is defined as follows:

CREATE TABLE T1(

INT1 INT,

XML1 XML,

XML2 XML );

Table 4-1 shows the data in table T1.

Table 4-1 Data in table T1

An application performs SQL read operations that are represented by the following

pseudocode:

EXEC SQL

DECLARE CURSOR C1 FOR

SELECT INT1, XML1

Support: If the base table space is not a universal table space, it does not support multiple

XML versions. To convert the base table space from either segmented or partitioned to

universal table space, you must remove it (with the DROP command) and re-create it.

ALTER and REORG are not sufficient in this case.

XML versions: XML versions differ from table space versions or index versions. The

purpose of XML versions is to optimize concurrency and memory usage. The purpose of

table space and index versions is to maximize data availability.

INT1 XML1 XML2

350 <A1>111</A1> <A2>aaa</A2>

100 <A1>111</A1> <A2>aaa</A2>

250 <A1>111</A1> <A2>aaa</A2>