AcuODBC User's Guide
5.4 The AcuODBC Optimizer
AcuODBC has a built-in optimizer that optimizes queries by selecting the most appropriate index to read the file.
The index is selected according to the conditions in the WHERE or ON clause, without considering the size of files. Conditions that specify a value for the primary key are especially efficient.
This allows AcuODBC to predict the order of the resulting table, even if the ORDER BY clause is not specified. ORDER BY forces AcuODBC to use a temporary file for ordering data. This slows performance.
If none of the fields in the WHERE or ON clause is indexed, the entire file must be read to determine which records obey the specified condition.
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