It returns an error when the source and destination names are the same, or 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.
OKredis> RENAME mykey myotherkey
OKredis> GET myotherkey