AcuODBC has a built-in optimizer which optimizes queries by selecting the most appropriate index to read the file.
The index is selected according to the conditions declared with the WHERE option, without considering the size of files.
This allows AcuODBC to predict the order of the resulting table, even if the ORDER BY option is not specified. ORDER BY forces AcuODBC to use a temporary file for ordering data. This slows performance.
To be a key field, a field must be used after the WHERE option to address a contiguous group of lines valid for the entire query. It does not have to be signed.
If none of the fields specified after the WHERE command satisfies the previous conditions (or if one has not been specified after the WHERE command), a sequential reading is done.