When should you NOT use an index?

Avoid indexes when: table is small (full scan is fast enough), columns have low selectivity (many duplicates - e.g., gender), table is heavily write-intensive (index maintenance overhead), columns are rarely queried, using expressions/functions on columns (unless using expression index), wide columns (large index size). Also avoid over-indexing - each index slows writes and uses storage. Monitor unused indexes and remove them.