What index is suitable if the column has few unique values and many NULL values?

If a column has few unique values and many NULLs, a regular B-Tree index will be inefficient, as it handles low selectivity and a large number of NULLs poorly.

The best options might be:

• Bitmap index (if supported by the DBMS), which works well with low cardinality and allows efficient filtering on multiple such columns.
• Partial index, indexing only non-NULL values, if supported by the DBMS.
• In some cases, a filtered index excluding NULLs can be used to reduce index size and improve efficiency.

The choice depends on the specific DBMS and the nature of the queries, but the key is to use an index optimized for low selectivity and many NULLs.