MySQL transaction isolation level details

serializable serialization (no problem)

Transactions must be executed in a sequential manner. The subsequent transactions cannot be committed before the previous transaction is committed. This is the safest method, but concurrent operations are not possible, resulting in low efficiency.

repeatab read repeatable read (default isolation level) (phantom read)

Before a transaction is committed, no matter how many queries are executed, the query results are the same (even if the record has been modified by other transactions), but phantom reads may occur.

read committed committed (non-repeatable, phantom read)

In the current transaction, data submitted by other transactions can be seen, which may lead to non-repeatable read (after another thread submits data, the current thread can see it, and then the results of the same SQL query twice before and after will be different (compared to repeatable read)).

Phantom reading may also occur
User1 queries wangwu and finds that it does not exist. Then user2 starts a transaction and inserts wangwu but does not commit the data. User1 queries again and still finds that it does not exist. The operation of inserting wangwu is executed, but the operation fails. Obviously, wangwu does not exist but cannot be inserted, resulting in phantom read.

read uncommitted uncommitted (phantom read, non-repeatable read, dirty read)

• Dirty read: The current transaction will read data that is not committed by other transactions. If other transactions are rolled back, the data read by the current transaction is illegal, which is called dirty read.
• Non-repeatable read occurs: modifications submitted by other transactions will be perceived by the current transaction, so the query results are different.
• Phantom read occurs: First, user 1 queries wangwu and it does not exist. User 2 starts a transaction and inserts wangwu but does not commit the transaction. At this time, user1 queries wangwu and finds that it exists.

The deletewangwu operation failed. wangwu was found but could not be deleted?

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