Lotte

The cause that caught me and is not on the list: the index was on the columns but in an order that did not match how the query filters. A composite index is usable from the left, so filtering on the second column alone does not touch it.

Same columns, same index, entirely unused. Order is not cosmetic.

A function or a cast on the indexed column silently disqualifies the index, and it is easy to add one without noticing.