MySQL and Oracle: Comparison of support for transaction isolation levels

MySQL and Oracle: Comparison of support levels for transaction isolation levels

With the rapid development of web applications and enterprise-level applications, the requirements for concurrent access and data consistency of databases are getting higher and higher. As an important function to ensure the execution of database transactions, transaction isolation level is particularly important for database concurrency control and data integrity. In database systems, MySQL and Oracle are two widely used relational database management systems (RDBMS). This article will focus on exploring their support for transaction isolation levels.

1. Introduction to transaction isolation level

Transaction isolation level refers to the degree of mutual influence between multiple concurrent transactions. The database management system determines whether to allow various concurrency problems between transactions based on the isolation level of the transaction, such as dirty read (Dirty Read), non-repeatable read (Non-Repeatable Read) and phantom read (Phantom Read).

The four common transaction isolation levels are:

• Read Uncommitted (Read Uncommitted): modifications in the transaction can be read by other transactions and may lead to dirty reads, Issues such as non-repeatable reads and phantom reads.
• Read Committed: Modifications in a transaction can only be read by another transaction, avoiding dirty read problems, but non-repeatable read and phantom read problems may occur.
• Repeatable Read: Modifications in a transaction can only be read by another transaction, avoiding dirty read and non-repeatable read problems, but phantom read problems may occur.
• Serializable: The highest isolation level, all transactions are executed in order, avoiding all concurrency problems, but it will affect system performance.

2. MySQL transaction isolation level support

The default transaction isolation level of MySQL is Repeatable Read (Repeatable Read), you can also set the session isolation level to modify. The transaction isolation levels supported by MySQL from low to high are: read uncommitted, read committed, repeatable read, and serialized.

The following is a sample code for setting MySQL's transaction isolation level to read committed:

SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;

3. Oracle's transaction isolation level support

Oracle's default transaction isolation level is Read Committed, which can also be modified by setting the isolation level of the transaction or setting the isolation level of the session. The transaction isolation levels supported by Oracle, from low to high, are: read uncommitted, read committed, repeatable read, and serialized.

The following is a sample code for setting Oracle's transaction isolation level to repeatable read:

ALTER SESSION SET ISOLATION_LEVEL READ COMMITTED;

4. Transaction isolation level comparison

MySQL and Oracle's support for transaction isolation levels is basically the same. They all support four transaction isolation levels, and the default isolation level can be changed by setting the session or transaction.

It should be noted that MySQL's transaction isolation level setting is effective for the current connection, while Oracle's transaction isolation level setting is effective for the current session.

In addition, MySQL and Oracle also have different solutions to concurrency problems caused by different transaction isolation levels. In MySQL, locks are usually used to solve concurrency problems, while in Oracle, a more complex data version control mechanism is used.

5. Summary

Transaction isolation level is one of the important mechanisms in the database management system to ensure concurrency control and data consistency. As two widely used relational databases, MySQL and Oracle both provide good support for transaction isolation levels.

When setting the transaction isolation level, it is necessary to comprehensively consider the trade-off between system performance and data consistency based on specific application scenarios and requirements. At the same time, developers also need to pay attention to the concurrency issues of database query and update operations, and reasonably choose the appropriate transaction isolation level to improve system concurrency and data integrity.

Reference link:

• [MySQL official document](https://dev.mysql.com/doc/refman/8.0/en/set-transaction.html)
• [Oracle official documentation](https://docs.oracle.com/cd/B28359_01/server.111/b28286/statements_10005.htm)

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