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SQL Anywhere 12.0.0 (Français) » UltraLite - Database Management and Reference » Using UltraLite databases » UltraLite performance and optimization

 

Tuning query performance with index hashing

You can tune the performance of your queries by choosing a specific size for the maximum hash. A hash key represents the actual values of the indexed column. An index hash key aims to avoid the expensive operation of finding, loading, and then unpacking the rows to determine the indexed value. It prevents these operations by including enough of the actual row data with a row ID.

A row ID allows UltraLite to locate the actual row data in the database file. If you set the hash size to 0 (which disables index hashing), then the index entry only contains this row ID. If you set the hash size to anything other than 0, then a hash key is also used. A hash key can contain all or part of the transformed data in that row, and is stored with the row ID in the index page.

How much row data the hash key includes is determined:

  • Partly by the maximum hash size property you configure. See Choosing an optimal hash size.

  • Partly by how much is actually needed for the data type of the column.

 A hash example
 See also