NewHashEqualer creates a new HashEqualer.

NilFlag and NotNilFlag are used to indicate whether a pointer/interface type field inside struct is nil or not.

NilFlag and NotNilFlag are used to indicate whether a pointer/interface type field inside struct is nil or not.

Equals is the interface for equality check.

Hash64 is the interface for hashcode.

HashEquals is the interface for hash64 and equality check.

Hasher is the interface for computing hash values of different types.

Scheduler is a scheduling interface defined for serializing(single thread)/concurrent(multi thread) running.

Stack is abstract definition of task container.(TaskStack is a kind of array stack implementation of it).

Task is an interface defined for all type of optimizing work: exploring, implementing, deriving-stats, join-reordering and so on.

Hash64a is the type for the hash value.