Database
Mysql Tutorial
What are the transaction isolation levels in MySQL, and which is the default?
What are the transaction isolation levels in MySQL, and which is the default?
Jun 23, 2025 pm 03:05 PM
MySQL's default transaction isolation level is Repeatable Read, which prevents dirty reads and non-repeatable reads through MVCC and gap locks, and avoids phantom reading in most cases; other major levels include Read Uncommitted, allowing dirty reads but the fastest performance, 1. Read Committed ensures that the submitted data is read but may encounter non-repeatable reads and phantom readings, 2. Repeatable Read default level, ensuring the consistency of multiple reads within the transaction, 3. Serialization is the highest level, preventing other transactions from modifying data through locks, ensuring data integrity but sacrificing performance; SELECT @@tx_isolation; and SET SESSION TRANSACTION ISOLATION LEVEL to view or set isolation levels.
MySQL supports several transaction isolation levels, which determine how transactions interact with each other and how data consistency is maintained. The default isolation level in MySQL is REPEATABLE READ .
Here's a breakdown of the main isolation levels you might encounter:
Read Uncommitted – Lowest Isolation
This level allows one transaction to read uncommitted changes made by another transaction. It can lead to "dirty reads," where data that hasn't been finalized (and might be rolled back) is read.
- Not commonly used in practice due to risk of inconsistency
- Fastest performance-wise, but least safe
Example:
If Transaction A updates a row but hasn't committed yet, Transaction B can see that update immediately—even if it gets rolled back later.
Read Committed – Avoids Dirty Reads
This level ensures that any data read during a transaction is committed at the moment it is read.
- Prevents dirty reads
- May still allow non-repeatable reads and phantom reads
In some databases like Oracle, this is the default, but not in MySQL.
Use case:
When you want to make sure you're only reading committed data, but don't need strict consistency across the entire transaction.
Repeatable Read – Default in MySQL
At this level, MySQL ensures that if a transaction reads the same data multiple times, it will see the same values ??throughout the transaction.
- Prevents dirty reads and non-repeatable reads
- Also prevents most phantom reads (though technically possible in some edge cases)
- Uses multi-version concurrency control (MVCC) and next-key locks to achieve this
Because it's the default, you'll often find it being used without explicit configuration.
Common behavior:
- If two transactions try to modify the same rows, one will wait or fail depending on locking
- Consistent reads within a transaction use a snapshot of the database
Serializable – Highest Isolation
This is the strictest isolation level. It places a lock on the rows being read, preventing other transactions from modifying them until the current transaction completes.
- Prevents dirty reads, non-repeatable reads, and phantom reads
- Can significantly reduce performance due to increased locking
It's typically used when absolute data integrity is required and performance isn't a top priority.
Summary of Isolation Levels and Behavior
| Isolation Level | Dirty Reads | Non-Repeatable Reads | Phantom Reads |
|---|---|---|---|
| Read Uncommitted | Possible | Possible | Possible |
| Read Committed | No | Possible | Possible |
| Repeatable Read | No | No | Rare* |
| Serializable | No | No | No |
*MySQL uses next-key locking in Repeatable Read to prevent most phantom reads.
You can check or change the isolation level using SQL commands like:
SELECT @@tx_isolation; SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;
Just keep in mind that changing the isolation level affects how your transactions behave under load and concurrency.
Basically that's it.
The above is the detailed content of What are the transaction isolation levels in MySQL, and which is the default?. For more information, please follow other related articles on the PHP Chinese website!
Hot AI Tools
Undress AI Tool
Undress images for free
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Clothoff.io
AI clothes remover
Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
Performing logical backups using mysqldump in MySQL
Jul 06, 2025 am 02:55 AM
mysqldump is a common tool for performing logical backups of MySQL databases. It generates SQL files containing CREATE and INSERT statements to rebuild the database. 1. It does not back up the original file, but converts the database structure and content into portable SQL commands; 2. It is suitable for small databases or selective recovery, and is not suitable for fast recovery of TB-level data; 3. Common options include --single-transaction, --databases, --all-databases, --routines, etc.; 4. Use mysql command to import during recovery, and can turn off foreign key checks to improve speed; 5. It is recommended to test backup regularly, use compression, and automatic adjustment.
Handling NULL Values in MySQL Columns and Queries
Jul 05, 2025 am 02:46 AM
When handling NULL values ??in MySQL, please note: 1. When designing the table, the key fields are set to NOTNULL, and optional fields are allowed NULL; 2. ISNULL or ISNOTNULL must be used with = or !=; 3. IFNULL or COALESCE functions can be used to replace the display default values; 4. Be cautious when using NULL values ??directly when inserting or updating, and pay attention to the data source and ORM framework processing methods. NULL represents an unknown value and does not equal any value, including itself. Therefore, be careful when querying, counting, and connecting tables to avoid missing data or logical errors. Rational use of functions and constraints can effectively reduce interference caused by NULL.
Aggregating data with GROUP BY and HAVING clauses in MySQL
Jul 05, 2025 am 02:42 AM
GROUPBY is used to group data by field and perform aggregation operations, and HAVING is used to filter the results after grouping. For example, using GROUPBYcustomer_id can calculate the total consumption amount of each customer; using HAVING can filter out customers with a total consumption of more than 1,000. The non-aggregated fields after SELECT must appear in GROUPBY, and HAVING can be conditionally filtered using an alias or original expressions. Common techniques include counting the number of each group, grouping multiple fields, and filtering with multiple conditions.
Paginating Results with LIMIT and OFFSET in MySQL
Jul 05, 2025 am 02:41 AM
MySQL paging is commonly implemented using LIMIT and OFFSET, but its performance is poor under large data volume. 1. LIMIT controls the number of each page, OFFSET controls the starting position, and the syntax is LIMITNOFFSETM; 2. Performance problems are caused by excessive records and discarding OFFSET scans, resulting in low efficiency; 3. Optimization suggestions include using cursor paging, index acceleration, and lazy loading; 4. Cursor paging locates the starting point of the next page through the unique value of the last record of the previous page, avoiding OFFSET, which is suitable for "next page" operation, and is not suitable for random jumps.
Implementing Transactions and Understanding ACID Properties in MySQL
Jul 08, 2025 am 02:50 AM
MySQL supports transaction processing, and uses the InnoDB storage engine to ensure data consistency and integrity. 1. Transactions are a set of SQL operations, either all succeed or all fail to roll back; 2. ACID attributes include atomicity, consistency, isolation and persistence; 3. The statements that manually control transactions are STARTTRANSACTION, COMMIT and ROLLBACK; 4. The four isolation levels include read not committed, read submitted, repeatable read and serialization; 5. Use transactions correctly to avoid long-term operation, turn off automatic commits, and reasonably handle locks and exceptions. Through these mechanisms, MySQL can achieve high reliability and concurrent control.
Calculating Database and Table Sizes in MySQL
Jul 06, 2025 am 02:41 AM
To view the size of the MySQL database and table, you can query the information_schema directly or use the command line tool. 1. Check the entire database size: Execute the SQL statement SELECTtable_schemaAS'Database',SUM(data_length index_length)/1024/1024AS'Size(MB)'FROMinformation_schema.tablesGROUPBYtable_schema; you can get the total size of all databases, or add WHERE conditions to limit the specific database; 2. Check the single table size: use SELECTta
Handling character sets and collations issues in MySQL
Jul 08, 2025 am 02:51 AM
Character set and sorting rules issues are common when cross-platform migration or multi-person development, resulting in garbled code or inconsistent query. There are three core solutions: First, check and unify the character set of database, table, and fields to utf8mb4, view through SHOWCREATEDATABASE/TABLE, and modify it with ALTER statement; second, specify the utf8mb4 character set when the client connects, and set it in connection parameters or execute SETNAMES; third, select the sorting rules reasonably, and recommend using utf8mb4_unicode_ci to ensure the accuracy of comparison and sorting, and specify or modify it through ALTER when building the library and table.
Setting up asynchronous primary-replica replication in MySQL
Jul 06, 2025 am 02:52 AM
To set up asynchronous master-slave replication for MySQL, follow these steps: 1. Prepare the master server, enable binary logs and set a unique server-id, create a replication user and record the current log location; 2. Use mysqldump to back up the master library data and import it to the slave server; 3. Configure the server-id and relay-log of the slave server, use the CHANGEMASTER command to connect to the master library and start the replication thread; 4. Check for common problems, such as network, permissions, data consistency and self-increase conflicts, and monitor replication delays. Follow the steps above to ensure that the configuration is completed correctly.
