Question about REPEATABLE READ isolatin level.

[Stephan Smith]

Hey guys,

From the description of this isolation level here: https://dev.mysql.com/doc/refman/8.4/en/innodb-transaction-isolation-levels.html, it says 'It is not recommended to mix locking statements (UPDATE, INSERT, DELETE, or SELECT ... FOR ...) with non-locking SELECT statements in a single REPEATABLE READ transaction'.

Does it mean that it is preferable that we feed it with READ-ONLY transactions or ones with only DML statements inside? I test it with the following transaction:

start transaction;

insert into t values (1, 1);

select * from t;

commit;

It commits successfully though. Is there something like a performance penalty if I do this in a production environment?

Stephan