![]() ![]() SELECT * FROM address_type WHERE id = 1 FOR UPDATE The exclusive lock or FOR UPDATE option allows reads but not lock reads or writes. To recap, the SHARE MODE lock can be acquired for more than one session, and it locks writes. SELECT * FROM zipcode WHERE code = 'SW6' FOR UPDATE Īnd then, we run the same SELECT statement with the SHARE MODE option used before in the first session, but now in the second one, and we’ll observe once more the timeout error. Besides that, we should remember that reads are allowed here.Īs opposed to the SHARE MODE, the FOR UPDATE doesn’t allow read locks, as shown next when we run a statement in the first session: SET autocommit=0 Now, we’ll run the same update as we did previously, and the result is again the timeout. SELECT * FROM zipcode WHERE code = 'SW6' LOCK IN SHARE MODE ![]() First, let’s use the shared mode: SET autocommit=0 However, we didn’t see how to define them explicitly using the LOCK IN SHARE MODE and FOR UPDATE options. We already discussed shared and exclusive locks. Let’s not forget to rollback both session transactions at the end of each test. 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.
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