The 'one index per query' is not correct. The number of indexes used will generally be limited to one each per table involved in the query. Note that something like:
```
select a,
b,
(
select sum(c)
from t
where a == o.a
)
from t as o
order by b
;
```
uses two tables and each one will use a different index (if available). The most efficient index for the outer query is t(b) (to be able to traverse the rows of t in order and avoid having to sort) and that the most efficient index for the correlated subquery will be a covering index t(a,c) to permit the appropriate traversal of the table in order and generating the sum without accessing the underlying table).
The CLI has a `.expert` command that will attempt to tell you what indexes, if available) would produce the least-cost solution.