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

With this SQL enhancement, a single FETCH can be used to retrieve multiple rows of
data, and an INSERT can insert one or more rows into a table. This reduces the number of
times the application and database must switch control. It can also reduce the number of
network trips required for multiple fetch or insert operations for distributed requests. For
some applications, this can help performance dramatically.
► RUNSTATS improvements
The improvements for RUNSTATS are related to the following areas:
– Code optimization
The changes have produced up to a 40% reduction in the execution time of a
RUNSTATS TABLESPACE.
– Distribution statistics
This enhancement adds new functionality of calculating frequencies for non-leading
and non-indexed columns to RUNSTATS. The relevant catalog tables are updated with
the specified number of highest frequencies and optionally with the specified number of
lowest frequencies. The new functionality also optionally collects multi-column
cardinality for non-indexed column groups and column groups of non-leading index
columns, and updates the catalog.
– Fast-cached SQL statement invalidation
This enhancement adds new functionality of allowing UPDATE NONE and REPORT
NO keywords to be used on the same RUNSTATS utility execution. This causes the
utility to only invalidate statements in the dynamic statement cache without any data
access or computation cost.
► Host variables’ impact on access paths
The enhancement allows for a new BIND option, REOPT(ONCE). It allows you to
re-optimize an SQL dynamic statement based on the host variable value, the first time it is
executed. After that, the statement is stored in the dynamic statement cache.
► Index only access for VARCHAR
This enhancement removes the key padding associated with VARCHAR index columns.
This allows the use of these columns to satisfy the results of queries that can use index
only access.
► Backward index scan
The backward index chain has been introduced with Type 2 indexes. However, only in V7
did DB2 start to exploit the backward index chain for MIN and MAX functions. In V8, DB2
extends this functionality with the capability for backward index scans. This allows DB2 to
avoid more sorts and allows customers to define fewer indexes.
► Local SQL cache issues and short prepare
This enhancement reduces the cost of a short prepare.
► Multiple IN values
This enhancement causes the DB2 optimizer to make a wiser choice when considering
index usage on single table SQL queries that involve large numbers of IN list items.
► Dynamic statement cache statement ID in EXPLAIN
This enhancement allows the DB2 EXPLAIN statement to report the chosen access path
of an SQL statement currently in the dynamic statement cache (DSC).
► Locking improvements
The most important improvements in locking:
– Reduced lock contention on volatile tables
Chapter 1. DB2 UDB for z/OS Version 8 7