IBM eserver i5 Handbook page 560

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performance for both centralized and distributed client/server database access,
making the iSeries the database server of choice for many computing needs.
The following DB2 UDB for iSeries functions are available to enhance application
performance:
Advanced SQL optimizer
This optimizer converts SQL requests into optimally efficient database access
methods, using proven mathematical rules and to query specific cost
estimates. Optimal performance is maintained over time by the automatic
rebind feature, which redetermines access methods based on changes to the
database objects and statistics. The optimizer detects changes in the number
of processors on each query. When processors are moved to an LPAR or
when Capacity Upgrade on Demand adds additional processors, the query
plan is automatically re-optimized if necessary.
SQL encoded vector indexes (EVIs)
An EVI can be created through SQL and improve query performance,
especially for long-running queries that run against large files using many
selection criteria. An EVI has several advantages over a traditional index with
the same keys, for example:
– Precise statistics about the distribution of key values are automatically
maintained and can be accessed quicker by the query optimizer than
traditional indexes.
– EVIs can be built much faster and take significantly less storage than
traditional indexes. Less storage means less main storage is necessary to
run the query.
– The query optimizer can scan EVIs and dynamically build bitmaps quicker
than from traditional indexes.
Explain function
The Explain function examines and reports the access method used by
individual SQL queries. The output can be examined to determine whether
the access method generated for the query can be improved by query or
database changes.
Block INSERT and FETCH
Block Insert and Fetch provides applications with the ability to store and
retrieve arrays of data directly, instead of one row at a time.
Automatic record blocking
An automatic recording blocking function improves client/server performance
by returning rows to the client in blocks rather than individually. Subsequent
IBM Eserver i5 and iSeries System Handbook
526
Draft Document for Review October 18, 2004