DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

1.1.3 Data warehouse
1.1.4 Performance
data sets are requested above the 16 MB line. When those open data sets (or partitions)
are compressed, the compression dictionary is loaded above the 2 GB bar.
► More log data sets
The maximum number of active log data sets per log copy is increased from 31 to 93. The
maximum number of archive log volumes recorded in the BSDS is increased from 1,000 to
10,000 per log copy. If this limit is exceeded, there is a wrap around, and the first entry
gets overwritten .
► CI size larger than 4 KB
DB2 V8 introduces support for CI sizes of 8, 16, and 32 KB. This is valid for user-defined
and DB2-defined table spaces. The new CI sizes relieve some restrictions on backup,
concurrent copy, and the use of striping, as well as provide the potential for reducing
elapsed time for large table space scans.
The most important enhancements specifically affecting the data warehouse applications are:
► More tables in joins
In DB2 V7 the number of tables in the FROM clause of a SELECT statement can be 225
for a star join. However, the number of tables that can be joined in other types of join is 15.
DB2 V8 allows 225 tables to be joined in all types of joins.
► Sparse index and in-memory work files for star join
The star join implementation in DB2 UDB for z/OS potentially has to deal with a large
number of work files, especially for a highly normalized star schema that can involve many
snowflakes, and the cost of the sorting of these work files can be very expensive. DB2 V8
extends the use of a sparse index (a dynamically built index pointing to a range of values)
to the star join work files and adds a new optional function of data caching on star join
work files. The decision to use the sparse index is made based on the estimation of the
cost of the access paths available.
► Common table expression and recursive SQL
DB2 V8 introduces the common table expression and recursive SQL function, which
extends the expressiveness of SQL and lets users derive the query result through
recursion. It is also a convenient way for users to express complex queries, since using
common table expressions instead of views saves both users and the system the work of
creating and dropping views.
► Materialized query tables
This enhancement provides a set of functions that allows DB2 applications to define,
populate, and make use of materialized query tables. Large data warehouses definitely
benefit from MQTs for queries against objects in an operational data store.
However several other changes to improve optimization techniques, improve statistics for
optimization, and better analysis of the access path, listed under performance, also benefit
warehousing applications. Even more additions that add to warehousing applications are
enhancements to the ability to use indexes, the new DSTATS option of RUNSTATS, the new
Visual Explain, and the online schema changes.
The main DB2 V8 improvements related to performance are:
► Multi-row INSERT and FETCH
6 DB2 UDB for z/OS Version 8 Performance Topics