Nonunique indexes

In some applications, most table updates can be confined to a single time period. You might be able to set up your system so that all updates are applied overnight or on specified dates. Additionally, when updates are performed as a batch, you can drop all nonunique indexes while you make updates and then create new indexes afterward. This strategy can improve performance.

Dropping nonunique indexes can have the following positive effects:
  • The updating program can run faster with fewer indexes to update. Often, the total time to drop the indexes, update without them, and re-create them is less than the time to update with the indexes in place. (For a discussion of the time cost of updating indexes, see Time costs of indexes.)
  • Newly made indexes are more efficient. Frequent updates tend to dilute the index structure so that it contains many partly full leaf pages. This dilution reduces the effectiveness of an index and wastes disk space.

As a time-saving measure, make sure that a batch-updating program calls for rows in the sequence that the primary-key index defines. That sequence causes the pages of the primary-key index to be read in order and only one time each.

The presence of indexes also slows down the population of tables when you use the LOAD statement or the dbload utility. Loading a table that has no indexes is a quick process (little more than a disk-to-disk sequential copy), but updating indexes adds a great deal of overhead.

To avoid this overhead, you can:

  1. Drop the table (if it exists).
  2. Create the table without specifying any unique constraints.
  3. Load all rows into the table.
  4. Alter the table to apply the unique constraints.
  5. Create the nonunique indexes.

If you cannot guarantee that the loaded data satisfies all unique constraints, you must create unique indexes before you load the rows. It saves time if the rows are presented in the correct sequence for at least one of the indexes. If you have a choice, make it the row with the largest key. This strategy minimizes the number of leaf pages that must be read and written.

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