It returns an error when
key does not exist.
newkey already exists it is overwritten, when this happens RENAME executes an implicit DEL operation, so if the deleted key contains a very big value it may cause high latency even if RENAME itself is usually a constant-time operation.
Note: Before Redis 3.2.0, an error is returned if source and destination names are the same.
OKredis> RENAME mykey myotherkey
OKredis> GET myotherkey