MySQL cannot use a composite index in a lookup in which the
WHERE condition doesn't include the columns forming a left-most prefix:
MySQL cannot use the index to perform lookups if the columns do not form a leftmost prefix of the index.
A quote from this answer on PostgreSQL caught my attention:
This is somewhat different in Oracle 11 which can sometimes also use columns that are not at the beginning of the index definition.
Under what circumstances can Oracle (at least in 11g) do a lookup without the left-most prefix columns existing in the query?