Improve caching of query plans

[huahaiy]

Our plan generation can be slow sometimes, so it is desirable to reuse plans as much as possible by caching them.

Caching keys of the plans consider both the query and the arguments. We can be smart about the caching keys. If the arguments do not introduce dramatic changes in the amount of data retrieved, we can still reuse the plan, even if the arguments are different from the cached ones.