An index provides an ordering (either ascending or descending) of a table's rows based on the values in one or more columns. When UltraLite optimizes a query, it scans existing indexes to see if one exists for the table(s) named in the query. If it can help UltraLite return rows more quickly, the index is used. If you are using the UltraLite Table API in your application, you can specify an index that helps determine the order in which rows are traversed.
Indexes can improve the performance of a query—especially for large tables. To see whether a query is using a particular index, you can check the execution plan with Interactive SQL.
Alternatively, your UltraLite applications can include PreparedStatement objects which have a method to return plans.
Multi-column indexes are sometimes called composite indexes. Additional columns in an index can allow you to narrow down your search, but having a two-column index is not the same as having two separate indexes. For example, the following statement creates a two-column composite index:
CREATE INDEX name ON Employees ( Surname, GivenName );
A composite index is useful if the first column alone does not provide high selectivity. For example, a composite index on Surname and GivenName is useful when many employees have the same surname. A composite index on EmployeeID and Surname would not be useful because each employee has a unique ID, so the column Surname does not provide any additional selectivity.
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