Can MySQL use multiple indexes for a single query?

Question

Imagine a table with multiple columns, say, id, a, b, c, d, e. I usually select by id, however, there are multiple queries in the client app that u

Answer 1

• Each SELECT in a query -- think UNION, subquery, derived table, etc -- is optimized separately. That is each might use different indexes.
• One SELECT can use multiple indexes in what it calls "index merge". This is rarely used.
• When the EXPLAIN says it is using Index merge (intersect), then it can almost always be improved by using a composite index containing the columns used by the intersected indexes.
• Index merge (union) is sometimes (rarely) used for an OR. Such can perhaps be improved by rewriting the query to be the UNION of two SELECTs.