feat(storage): add is_sorted support for in-memory engine

[skyzh]

Signed-off-by: Alex Chi iskyzh@gmail.com

This PR adds is_sorted to the planner. If order by is applied on pk, we do not need an extra order by executor, as the storage layer has already got it sorted.

ref #114

[wangrunji0408]

LGTM
btw, why not add an interface to storage layer to report whether itself supports the sort-primary-key feature. If the planner sees this feature is not supported, then just fallback to the order executor. This way, for non-sorted storage like in-memory engine, we don't need to implement sorting again.

[skyzh]

LGTM
btw, why not add an interface to storage layer to report whether itself supports the sort-primary-key feature. If the planner sees this feature is not supported, then just fallback to the order executor. This way, for non-sorted storage like in-memory engine, we don't need to implement sorting again.

Indeed I have a plan to do so, but the planner must have access to the StorageImpl struct before doing this. We may take this into consideration when we implement the optimizer.