Let's just run the SELECT statement in the data_lock table to demonstrate what happened. Once again, transaction two timed out as the first one had acquired the lock of the rows in the table. UPDATE zipcode SET code = 'SW6 1AA' WHERE code = 'SW6' The next step is to run the following statement in the second session: SET autocommit=0 Here, we created a table and started a transaction that reads from the zipcode table and writes to the zipcode_backup table in a single statement. The number of locked rows depends on the statement execution plan, but we should keep in mind that all the tables involved may have some rows locked.Īs an example, let's rollback all the other transactions and execute these statements: CREATE TABLE zipcode_backup SELECT * FROM zipcode ĭELETE FROM zipcode_backup WHERE code IN (SELECT code FROM zipcode) Other common cases for the lock timeout error are DELETE and UPDATE statements involving multiple tables. However, later we can run the tests for all the other levels. First, in order to keep it short, we'll run all testing in the default isolation level REPEATABLE READ. Now that we understand how the different isolation levels work, let's run some tests to examine locking scenarios. SERIALIZABLE: behaves exactly like the previous one except that if autocommit is disabled, it locks the row during any update or delete, and reads are only allowed after commit.Even if another transaction changes the row right after the read, the baseline will always be returned if there are no changes after the first query REPEATABLE READ: the first query defines a snapshot, and it becomes the baseline for that row.READ COMMITTED: only committed changes are visible to other transactions.READ UNCOMMITTED : in short, all transactions can read all the changes made by others even if they were not committed. They are, respectively, from the least strict to the most: They provide different levels of balance between performance, consistency, reliability, and reproducibility. MySQL InnoDB offers four transaction isolation levels.
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