The question naturally arising now is, when is a bitmap scan most beneficial and when is it chosen by the optimizer? From my point of view, there are really only two use cases:
- Avoiding using the same block over and over again
- Combining relatively bad conditions
The first case is quite common. Suppose you are looking for everybody who speaks a certain language. For the sake of the example, we can assume that 10% of all people speak the required language. Scanning the index would mean that a block in the table has to be scanned all over again as many skilled speakers might be stored in the same block. By applying a bitmap scan, it is ensured that a specific block is only used once, which of course leads to ...
