MySQL and Oracle: Comparison of support for multi-version concurrency control and data consistency

MySQL and Oracle: Comparison of support for multi-version concurrency control and data consistency

Introduction:
In today's data-intensive applications, the database system plays a core role in realizing data storage and manage. MySQL and Oracle are two well-known relational database management systems (RDBMS) that are widely used in enterprise-level applications. In a multi-user environment, ensuring data consistency and concurrency control are important functions of the database system. This article will share the comparison of support between MySQL and Oracle in terms of multi-version concurrency control and data consistency, and attach code examples for explanation.

1. Multiversion Concurrency Control (MVCC)
Multiversion Concurrency Control (MVCC) is a method of handling concurrent access by assigning an independent historical version to each transaction. Achieve database consistency. MVCC allows multiple transactions to read the database simultaneously without conflicts. Below we will look at MySQL and Oracle's support for MVCC respectively.

1. MVCC in MySQL
   MySQL uses a row-based MVCC mechanism. The core idea is: for each data row, create a new version when modified and save the historical value. . This way, read operations are not blocked by write operations, improving concurrency performance. MySQL implements MVCC by storing hidden fields in data rows. For example, each data row in the InnoDB storage engine contains a 6-byte hidden field that records the creation timestamp and deletion timestamp. In this way, when each transaction reads data, it can determine the visibility of the data based on the timestamp.

Sample code:
Create test table:

CREATE TABLE test (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  age INT
) ENGINE=InnoDB;

Execute transaction 1 and transaction 2:

-- 事务1
START TRANSACTION;
SELECT * FROM test WHERE id = 1;
-- 执行一些其他操作
COMMIT;

-- 事务2
START TRANSACTION;
UPDATE test SET age = 20 WHERE id = 1;
-- 执行一些其他操作
COMMIT;

In MySQL, the above code can be executed concurrently without There will be no conflict. The data read by transaction 1 is the version before modification by transaction 2.

2. MVCC in Oracle
   Oracle uses a snapshot-based MVCC mechanism to ensure that the transaction is executed by creating a snapshot at the beginning of the transaction and releasing the snapshot at the end of the transaction. Execute within a consistent view. Oracle's snapshot uses a mechanism called UNDO (Undo Logs) to record old version data of transactions. When other transactions read data, Oracle will select an appropriate snapshot based on the transaction start time to ensure data consistency.

Sample code:
Create test table:

CREATE TABLE test (
  id INT PRIMARY KEY,
  name VARCHAR(50),
  age INT
);

INSERT INTO test VALUES (1, '张三', 18);

Execute transaction 1 and transaction 2:

-- 事务1
SET TRANSACTION READ ONLY;
SELECT * FROM test WHERE id = 1;
-- 执行一些其他操作

-- 事务2
BEGIN
  UPDATE test SET age = 20 WHERE id = 1;
  -- 执行一些其他操作
COMMIT;

In Oracle, the above code can be executed concurrently without There will be no conflict. The data read by transaction 1 is the version before modification by transaction 2.

2. Comparison of data consistency support
On the basis of ensuring multi-version concurrency control, the database system also needs to provide consistency guarantees. Below we will compare MySQL and Oracle's support for data consistency.

1. Data consistency in MySQL
   In MySQL, data consistency is provided by using transactions and locking mechanisms. Transactions can combine multiple operations into a logical unit and require that these operations either all execute successfully or all be rolled back. MySQL provides ACID (Atomicity, Consistency, Isolation and Durability) features to ensure data consistency. For example, use the BEGIN, ROLLBACK, and COMMIT statements to control the start, rollback, and commit of a transaction.

Sample code:

BEGIN;
-- 执行一些操作
ROLLBACK; -- 或者COMMIT;

In MySQL, the start and end of a transaction are controlled through the BEGIN and COMMIT or ROLLBACK statements to ensure the consistency of data operations.

2. Data consistency in Oracle
   Oracle provides more stringent transaction isolation levels, including Read Committed, Serializability and Serializable. At higher isolation levels, Oracle can provide stronger consistency guarantees. For example, the Serializability isolation level prohibits any concurrent operations and serializes transactions to achieve the highest level of consistency.

Sample code:

SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN;
-- 执行一些操作
ROLLBACK; -- 或者COMMIT;

In Oracle, adjust the consistency requirements of the data by setting the isolation level of the transaction. Higher isolation levels can improve consistency guarantees, but may sacrifice certain concurrency performance.

Conclusion:
MySQL and Oracle provide different support in terms of multi-version concurrency control and data consistency. MySQL uses a row-based MVCC mechanism to implement multi-version control of data through timestamps, and provides ACID features to ensure data consistency. Oracle uses a snapshot-based MVCC mechanism and provides a strict transaction isolation level to achieve a higher level of data consistency. When choosing a database system, you need to weigh which database system to use based on specific application scenarios and performance requirements.

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