A map representing positions within the supplied {@link #getSQL query} to
which we need to bind named parameters.
Optional, may return null if no named parameters.
The structure of the returned map (if one) as follows:
- The keys into the map are the named parameter names
- The corresponding value is either an {@link Integer} if the
parameter occurs only once in the query; or a List of Integers if the
parameter occurs more than once
