Use indexes

This is because the Optimizer uses them for performance optimization. Indexes are created in five different ways:

• CREATE INDEX (in SQL)
• CRTPF, with key
• CRTLF, with key
• CRTLF, as join logical file
• CRTLF, with select/omit specifications, without a key, and without dynamic selection (DYNSLT).

Indexes are used to enable row selection by means of index-versus-table scanning, which is usually slower. Table scanning sequentially processes all rows in a table. If a permanent index is available, building a temporary index can be avoided. Indexes are required for:

• Join tables
• ORDER BY
• GROUP BY

Indexes will be created, if no permanent index exists.

Manage the number of indexes to minimize the extra server cost of maintaining the indexes during update operations. Below are general rules for particular types of tables:

Primarily read-only tables:

Create indexes over columns as needed. Consider creating an index only if a table is greater than approximately 1,000 rows or is going to be used with ORDER BY, GROUP BY, or join processing. Index maintenance could be costlier than occasionally scanning the entire table.

Primarily read-only table, with low update rate:

Create indexes over columns as needed. Avoid building indexes over columns that are updated frequently. INSERT, UPDATE, and DELETE will cause maintenance to all indexes related to the table.

High update-rate tables:

Avoid creating many indexes. An example of a table that has a high update rate is a logging or a history table.