When designing indexes, consider the query patterns first. A composite index should match the WHERE clause order. Also, avoid over-indexing; each index adds write overhead.
Good point, Jane! Also, remember that covering indexes can eliminate the need to read the table data entirely, which can be a huge performance win for read‑heavy workloads.
Don't forget about index maintenance. Regularly rebuild or reorganize fragmented indexes, especially on tables with high insert/delete activity.