In general, MySQL uses an index when it is reasonably confident that doing so is more efficient than not doing so. This leads to false negatives during testing. The false negative tends to occur normally in the two situations which are Too Little Diversity and Index-Based Ordering.
Re: False negatives
In general, MySQL uses an index when it is reasonably confident that doing so is more efficient than not doing so. This leads to false negatives during testing. The false negative tends to occur normally in the two situations which are Too Little Diversity and Index-Based Ordering.