0203: Update Modes

We can now map SQL operations to KV. For example, select → get, delete → del.

But the KV set operation seems to be both insert and update. In fact, it is insert ... on duplicate update. This is a common use case. PostgreSQL adds an upsert statement to match the KV set operation.

We need to add insert and update behavior to KV. Add SetEx():

type UpdateMode int

const (
    ModeUpsert UpdateMode = 0 // insert or update
    ModeInsert UpdateMode = 1 // insert new
    ModeUpdate UpdateMode = 2 // update existing
)

func (kv *KV) SetEx(key []byte, val []byte, mode UpdateMode) (bool, error)

Requirements:

• ModeInsert: if the key already exists, do not update and return false.
• ModeUpdate: only update existing keys.
• ModeUpsert: same as the old Set(), insert or overwrite:

func (kv *KV) Set(key []byte, val []byte) (updated bool, err error) {
    return kv.SetEx(key, val, ModeUpsert)
}