This is mainly a conjecture on my part, but the technical reason stems from when a
SERIALIZABLE transaction goes into effect.
According to Paragraph 4 of the MySQL 5.7 Documentation on Transaction Isolation Levels
You can enforce a high degree of consistency with the default REPEATABLE READ level, for operations on crucial data where ACID compliance is important.
SERIALIZABLE enforces even stricter rules than REPEATABLE READ, and is used mainly in specialized situations, such as with XA transactions and for troubleshooting issues with concurrency and deadlocks.
How can you make
SERIALIZABLE enforce consistency rules stricter than
REPEATABLE READ ? Note the definition of
SERIALIZABLE from the same Documentation Page:
This level is like REPEATABLE READ, but InnoDB implicitly converts all plain SELECT statements to SELECT ... LOCK IN SHARE MODE if autocommit is disabled. If autocommit is enabled, the SELECT is its own transaction. It therefore is known to be read only and can be serialized if performed as a consistent (nonlocking) read and need not block for other transactions. (To force a plain SELECT to block if other transactions have modified the selected rows, disable autocommit.)
So, you have to experiment with autocommit to see the differences.
With autocommit disabled used in
SELECT ... LOCK IN SHARE MODE is summoned to bring about some readable locking but the locks would appear preemptively in a
REPEATABLE READ at certain stages before a read takes place.
With autocommit enabled,
SERIALIZABLE can be viewed as being like
REPEATABLE READ because nonlocking reads can run wild and the data being read would be essentially be the same.
When you look at transactions from this perspective,
SERIALIZABLE can never be trusted to be consistent because of the stagnating shared locks. This really disqualifies it from being consistent. That is why
REPEATABLE READ is the only isolation level used for ACID compliance.
Therefore, any snapshot made
SERIALIZABLE could look different at different points in time as opposed to
REPEATABLE READ .