May 14, 2025 • Written by ROMAN AGABEKOV
This article is about the MySQL configuration parameter join_buffer_size. It explains that join_buffer_size is allocated per connection, so increasing it globally can waste memory and degrade performance if set too high. Instead of raising join_buffer_size, the article recommends adding indexes to optimize joins. If necessary, join_buffer_size can be adjusted per session or per query to handle large join operations more efficiently.
Basic Details
The join_buffer_size variable sets the buffer size for join queries that require a full table scan because they cannot use an index.
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Releem automatically tunes join_buffer_size and 44 other variables to improve MySQL performance. Try Releem for Free, or deepen your understanding by reading our detailed article.
join_buffer_size – Usage
The join_buffer_size variable can be adjusted to set the buffer cache size when adding in indexes doesn't work. Indexes are used to improve query times by helping MySQL find rows with specific column values more quickly. A full table scan, starting with the first row, is used without indexes.
The join buffer is allocated when the server needs a full table join with no indexes. After the query is complete, the memory allocation is freed. In 64-bit platforms, Windows shortens join_buffer_size values above 4GB to 4GB with a warning.
The join buffer is allocated when the server needs a full table join with no indexes. After the query is complete, the memory allocation is freed. In 64-bit platforms, Windows shortens join_buffer_size values above 4GB to 4GB with a warning.
join_buffer_size – Configuration
The join_buffer_size variable can be configured dynamically or globally, using the following configuration methods:
Configuration File:
[mysqld]
join_buffer_size = XX
Replace XX with a value to suit your database needs. Must restart MySQL servers to see changes applied.
Command Line:
mysqld> set global join_buffer_size = XX
Replace XX with a value to suit your database needs. This change will only be applied to new connections. Make a new connection to the server and the values will update.
SET GLOBAL will not persist through a server restart.
Configuration File:
[mysqld]
join_buffer_size = XX
Replace XX with a value to suit your database needs. Must restart MySQL servers to see changes applied.
Command Line:
mysqld> set global join_buffer_size = XX
Replace XX with a value to suit your database needs. This change will only be applied to new connections. Make a new connection to the server and the values will update.
SET GLOBAL will not persist through a server restart.
join_buffer_size – Considerations
Join_buffer_size default is 256KB, while the maximum is 4GB.
Because join_buffer_size is allocated per connection, MySQL is allocating the set buffer size value, times the number of max connections, at startup. The default join_buffer_size rarely needs to be increased, as it wastes server memory without helping performance.
You may induce significant performance degradation if the join_buffer cache size is larger than the queries that need it. Instead of trying to increase join_buffer_size to improve performance, consider adding indexes for your joins. MySQL can log queries that don't contain indexes for easy identification. Then these queries can have indexes added or modified as necessary.
A common recommendation for join_buffer_size is 1% of available RAM. If adding indexes is not possible, instead of changing the value of join_buffer_size globally, change it for a session when you anticipate when large join queries. You can also change the value of join_buffer_size on a per-query basis if it's only needed for a handful of queries.
Because join_buffer_size is allocated per connection, MySQL is allocating the set buffer size value, times the number of max connections, at startup. The default join_buffer_size rarely needs to be increased, as it wastes server memory without helping performance.
You may induce significant performance degradation if the join_buffer cache size is larger than the queries that need it. Instead of trying to increase join_buffer_size to improve performance, consider adding indexes for your joins. MySQL can log queries that don't contain indexes for easy identification. Then these queries can have indexes added or modified as necessary.
A common recommendation for join_buffer_size is 1% of available RAM. If adding indexes is not possible, instead of changing the value of join_buffer_size globally, change it for a session when you anticipate when large join queries. You can also change the value of join_buffer_size on a per-query basis if it's only needed for a handful of queries.
With Releem, you don't have to worry about any of the nuances of join_buffer_size or other variables. Releem will assess your server and provide a MySQL Performance Score and make configuration recommendations. Releem can automatically apply the recommended settings to improve MySQL Performance Score. And the whole time you don't have to do anything.
FAQ:
MySQL join_buffer_size Variable
MySQL join_buffer_size Variable
What is join_buffer_size in MySQL?
join_buffer_size is a MySQL variable that determines the size of the buffer used for joins that cannot use indexes. It is especially important for optimizing queries involving full table scans or large joins.
Why is join_buffer_size important for MySQL performance?
join_buffer_size affects the efficiency of join operations in MySQL. A poorly configured buffer size can lead to slower query execution and higher memory usage.
What is the default value of join_buffer_size in MySQL?
The default value of join_buffer_size varies depending on the MySQL version but is typically set to 256KB.
How can I change the join_buffer_size in MySQL?
You can change the join_buffer_size dynamically using the following command:
SET GLOBAL join_buffer_size = <value>;
To make the change permanent, update the MySQL
configuration file (my.cnf):
[mysqld]
join_buffer_size = <value>
What is the recommended value for join_buffer_size?
The recommended value depends on your server’s workload and available memory. Start with the default value and incrementally increase it while monitoring performance.
How does join_buffer_size affect memory usage?
Each thread performing a join allocates a separate join buffer. Setting the join_buffer_size too high can lead to excessive memory usage on servers handling many concurrent connections.
What is the relationship between join_buffer_size and query optimization?
Queries that perform full table scans or complex joins without proper indexing benefit from a larger join_buffer_size. However, efficient indexing usually reduces the need for a large join buffer.
How can I monitor the impact of join_buffer_size changes?
Use tools like Releem Query Analytics, MySQL Performance Schema or slow query logs to analyze query execution times before and after changing join_buffer_size.
Does increasing join_buffer_size always improve performance?
Not always. While a larger buffer can help in some scenarios, it can also waste memory or degrade performance if not carefully tuned.
Are there any alternative MySQL settings that impact join performance?
Yes, variables like sort_buffer_size and proper indexing play a significant role in optimizing join performance alongside join_buffer_size.
How does join_buffer_size compare to sort_buffer_size?
While join_buffer_size is used for joins, sort_buffer_size is used for sorting operations. Both are important for query optimization but serve different purposes.
What are the common mistakes when configuring join_buffer_size?
Common mistakes include setting the buffer size too high for high-concurrency environments or ignoring proper indexing in favor of increasing buffer sizes.
join_buffer_size is a MySQL variable that determines the size of the buffer used for joins that cannot use indexes. It is especially important for optimizing queries involving full table scans or large joins.
Why is join_buffer_size important for MySQL performance?
join_buffer_size affects the efficiency of join operations in MySQL. A poorly configured buffer size can lead to slower query execution and higher memory usage.
What is the default value of join_buffer_size in MySQL?
The default value of join_buffer_size varies depending on the MySQL version but is typically set to 256KB.
How can I change the join_buffer_size in MySQL?
You can change the join_buffer_size dynamically using the following command:
SET GLOBAL join_buffer_size = <value>;
To make the change permanent, update the MySQL
configuration file (my.cnf):
[mysqld]
join_buffer_size = <value>
What is the recommended value for join_buffer_size?
The recommended value depends on your server’s workload and available memory. Start with the default value and incrementally increase it while monitoring performance.
How does join_buffer_size affect memory usage?
Each thread performing a join allocates a separate join buffer. Setting the join_buffer_size too high can lead to excessive memory usage on servers handling many concurrent connections.
What is the relationship between join_buffer_size and query optimization?
Queries that perform full table scans or complex joins without proper indexing benefit from a larger join_buffer_size. However, efficient indexing usually reduces the need for a large join buffer.
How can I monitor the impact of join_buffer_size changes?
Use tools like Releem Query Analytics, MySQL Performance Schema or slow query logs to analyze query execution times before and after changing join_buffer_size.
Does increasing join_buffer_size always improve performance?
Not always. While a larger buffer can help in some scenarios, it can also waste memory or degrade performance if not carefully tuned.
Are there any alternative MySQL settings that impact join performance?
Yes, variables like sort_buffer_size and proper indexing play a significant role in optimizing join performance alongside join_buffer_size.
How does join_buffer_size compare to sort_buffer_size?
While join_buffer_size is used for joins, sort_buffer_size is used for sorting operations. Both are important for query optimization but serve different purposes.
What are the common mistakes when configuring join_buffer_size?
Common mistakes include setting the buffer size too high for high-concurrency environments or ignoring proper indexing in favor of increasing buffer sizes.