Intel AS/400 RISC Server - Manual (Page 52)

SQE for V5R2 Summary
Enhancements to DB2 for i5/OS, called SQE, were made in V5R2. The SQE enhancements are object
oriented implementations of the SQE optimizer, the SQE query engine and the SQE database statistics. In
V5R2 a subset of the read-only SQL queries will be optimized and run with the SQE enhancements. The
effect of SQE on performance will vary by workload and configuration. For the most recent information
on SQE please see the SQE web page on the DB2 for i5/OS web site located at
www.iseries.ibm.com/db2/sqe.html. More information on SQE for V5R2 is also available in the V5R2
redbook Preparing for and Tuning the V5R2 SQL Query Engine.
4.5 Indexing
Index usage can dramatically improve the performance of DB2 SQL queries. For detailed information on
using indexes see the white paper Indexing Strategies for DB2 for i5/OS at
http://www-1.ibm.com/servers/enable/site/education/abstracts/indxng_abs.html . The paper provides basic
information about indexes in DB2 for i5/OS, the data structures underlying them, how the system uses
them and index strategies. Also discussed are the additional indexing considerations related to
maintenance, tools and methods.
Encoded Vector Indices (EVIs)
DB2 for i5/OS supports the Encoded Vector Index (EVI) which can be created through SQL. EVIs
cannot be used to order records, but in many cases, they can improve query performance. An EVI has
several advantages over a traditional binary radix tree index.
y
The query optimizer can scan EVIs and automatically build dynamic (on-the-fly) bitmaps much more
quickly than from traditional indexes.
y EVIs can be built much faster and are much smaller than traditional indexes. Smaller indexes require
less DASD space and also less main storage when the query is run.
y
EVIs automatically maintain exact statistics about the distribution of key values, whereas traditional
indexes only maintain estimated statistics. These EVI statistics are not only more accurate, but also
can be accessed more quickly by the query optimizer.
EVIs are used by the i5/OS query optimizer with dynamic bitmaps and are particularly useful for
advanced query processing. EVIs will have the biggest impact on the complex query workloads found in
business intelligence solutions and ad-hoc query environments. Such queries often involve selecting a
limited number of rows based on the key value being among a set of specific values (e.g. a set of state
names).
When an EVI is created and maintained, a symbol table records each distinct key value and also a
corresponding unique binary value (the binary value will be 1, 2, or 4 bytes long, depending on the
number of distinct key values) that is used in the main part of the EVI, the vector (array). The subscript
of each vector (array) element represents the relative record number of a database table row. The vector
has an entry for each row. The entry in each element of the vector contains the unique binary value
corresponding to the key value found in the database table row.
IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
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Copyright IBM Corp. 2008 Chapter 4 - DB2 Performance
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