Bitmap indexes can include rows that have NULL values, unlike most other types of
indexes. Indexing of nulls can be useful for some types of SQL statements, such as
queries with the aggregate function COUNT.

位图索引与空值
1. 与其他大多数索引不同，位图索引、可以包含键值为NULL的行。
2. 将键值为空的行进行索引对有些 SQL 语句是有用处的，例如包含COUNT聚合函数的查询。