Recommendations for EVI use

Encoded vector indexes are a powerful tool for providing fast data access in decision support and query reporting environments, but to ensure the effective use of EVIs, you should implement EVIs with the following guidelines:

Create EVIs on

Read-only tables or tables with a minimum of INSERT, UPDATE, DELETE activity.
Key columns that are used in the WHERE clause - local selection predicates of SQL requests.
Single key columns that have a relatively small set of distinct values.
Multiple key columns that result in a relatively small set of distinct values.
Key columns that have a static or relatively static set of distinct values.
Non-unique key columns, with many duplicates.

Create EVIs with the maximum byte code size expected

Use the "WITH n DISTINCT VALUES" clause on the CREATE ENCODED VECTOR INDEX statement.
If unsure, use a number greater than 65,535 to create a 4 byte code, thus avoiding the EVI maintenance overhead of switching byte code sizes.

When loading data

Drop EVIs, load data, create EVIs.
EVI byte code size will be assigned automatically based on the number of actual distinct key values found in the table.
Symbol table will contain all key values, in order, no keys in overflow area.

Consider SMP and parallel index creation and maintenance

Symmetrical Multiprocessing (SMP) is a valuable tool for building and maintaining indexes in parallel. The results of using the optional SMP feature of i5/OS™ are faster index build times, and faster I/O velocities while maintaining indexes in parallel. Using an SMP degree value of either *OPTIMIZE or *MAX, additional multiple tasks and additional server resources are used to build or maintain the indexes. With a degree value of *MAX, expect linear scalability on index creation. For example, creating indexes on a 4 processor server can be 4 times as fast as a 1 processor server.

Checking values in the overflow area

You can also use the Display File Description (DSPFD) command (or iSeries™ Navigator - Database) to check how many values are in the overflow area. Once the DSPFD command is issued, check the overflow area parameter for details on the initial and actual number of distinct key values in the overflow area.

Using CHGLF to rebuild an index's access path

Use the Change Logical File (CHGLF) command with the attribute Force Rebuild Access Path set to YES (FRCRBDAP(*YES)). This command accomplishes the same thing as dropping and recreating the index, but it does not require that you know about how the index was built. This command is especially effective for applications where the original index definitions are not available, or for refreshing the access path.