unique_key_max_encoded_size
Maximum size (in bytes) of the order-preserving binary encoding of a singleUNIQUE KEY row.
unique_key_probe_implementation
Selects the UNIQUE KEY write-path probe implementation. Currently only a single-threaded baseline exists, soauto and simple both resolve to it.
Reserved: this setting has no effect yet — the value is not read until the UNIQUE KEY INSERT
write path that constructs the probe is wired up. Declared now so the lever ships with the
implementation.