The basic idea is that if the doc is

and the query is

then we should reject on the basis of 'a' before evaluating the $where clause (slow).

In the old Matcher, $and checks:

• simple operators eg equality, $gt, $in
• then regular expressions
• then nested compound expressions (nested $and, $or, etc)
• then $where

We can change the heuristics in the new implementation, but they should stay at least as good as the old ones.