I have a typical Java Spring + Postgres environment (the project is legacy). A persistence layer of backend has a mix of declared isolation levels - some of them are the default i.e. READ COMMITED, others are REPEATABLE READ and there are some which are SERIALIZABLE.
Sometimes, the same DB tables are accessed from parallel transactions with different isolation levels.
Are there some strict rules for such transactions' interactions?
I can, more or less, understand situation, when all the transactions have the same isolation level and some of them declare explicit locks in sake of fine-grained avoiding of undesirable read phenomenas, but not the case described above.