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MySQL sort_buffer_size

By Aanchal SharmaAanchal Sharma

MySQL sort_buffer_size

Definition of MySQL sort_buffer_size

MySQL sort_buffer_size is a parameter specified in MySQL server which is far from noticeable to regulate. It is each session buffer that is assigned each time it is required. The difficulty with the buffer derives from the way Linux assigns memory. Earlier Monty Taylor have defined the core concern in detail but essentially beyond 256kB the behavior modifies and becomes leisurelier. For MySQL sort_buffer_size, the memory is allocated per connection or thread. So, if the buffer memory is set to overhead 256kB then, it implements mmap() in place of malloc() for assignment of memory. In fact, this can be tuneable but the default remains 256kB.

Syntax:

We can illustrate the following syntax to query the command code for MySQL sort_buffer_size as follows:

SET VARIABLEASSIGNMENT [, VARIABLEASSIGNMENT] …
VARIABLEASSIGNMENT:
USER_VARIABLE_NAME = EXPRESSION
| [GLOBAL | SESSION] SYSTEM_VARIABLE_NAME = EXPRESSION
| [@@global. | @@session. | @@]SYSTEM_VARIABLE_NAME = EXPRESSION;

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Here, the SET query statement allocates values to several kinds of variables which hamper the operation of the MySQL server or the client. In the elder versions of MySQL you can see the SET OPTION was employed but in this syntax structure you will deplored in favour of SET without having the OPTION keyword.

It explains the uses of SET keyword for providing values to user variables or system variables. A user variable has the following syntax written and set as:

SET @VARIABLE_NAME = EXPRESSION;

Various system variables are noticed as dynamic and further can be altered while the MySQL server executes by applying the SET statement. For this we need to refer the SET as VARIABLE_NAME which is headed by a modifier optionally.

Also, to specify that a variable is global externally, precede its name using GLOBAL or @@global but a user should have the SUPER privilege to make the change.

Again, to state that a variable is session externally, precede its name using SESSION or @@session or @@. Since, this session variable change does not need any special rights but a user or client is able to modify only its own session variables not of any other ones.

Also, @@local and LOCAL keywords in the syntax can be applied as these denotes synonyms for @@session and SESSION. But when no modifier is available then the SET statement command modifies the session variable.

How does sort_buffer_size Function work in MySQL?

Every session executing a sort allocates a buffer memory space with this quantity of memory i.e. sort_buffer_size. It is not detailed to any storing engine. But when the status variable i.e. sort_merge_passes, is too great then a user requires to look at either cultivating the query indexes or growing this.

You may deliberate decreasing where there can be various minor sorts, like OLTP and incrementing where required by session. The optional least possible is 16k. The command line is denoted by: –sort-buffer-size=#. The scope of this MySQL sort_buffer_size has identifiers: Global or, Session and it is dynamic with data type number and holds the default value as 2097152(2M).

Examples

Let us view some query commands statements that defines the usage of MySQL sort_buffer_size and how to apply in the MySQL server as follows:

SET GLOBAL sort_buffer_size = 1000000, SESSION sort_buffer_size = 1000000;
SET @@GLOBAL.sort_buffer_size = 1000000, @@LOCAL.sort_buffer_size = 1000000;
SET GLOBAL max_connections = 1000, sort_buffer_size = 1000000;

The above syntax structure contains the SET command that can consist more than one variable assignments which is parted by commas. When various system variables are set, then the utmost current SESSION or GLOBAL modifier present in the SQL statement is implemented for the above syntax variables which contain no modifier stated.

When the user allocates system variable a value along with SET then, there suffix letters cannot be used in the value but can be applied with the start-up choice. Although we can generate the form of an expression for the value as below:

SET sort_buffer_size = 10*1024*1024;

On the other side, for compatibility, the @@VAR_NAME syntax is supported for system variable with few other database systems. If the session system variable is changed, this value rests in effect till the session ends or till the variable is changed to a different value. But the modification is not noticeable to other clients.

Now, when the global system variable is modified, then the value is recalled and implemented for fresh connections till the server resumes. But if you want to make the global system variable set to permanent then, you need to put it in the option file. Any client that has access to the global system variable can view the changes. Nevertheless, this change disturbs the equivalent session variable only for clients which are connected after the change. Hence, the change in the global variable for any client does not hamper the session variable that is presently connected and not even that of the client which concerns the SET GLOBAL command statement.

For avoiding inappropriate usage, the server MySQL generates an error when a user implements SET GLOBAL command with a variable which can only be applied with SET SESSION or when GLOBAL (or @@global) is not stated while we set a global variable. In order to set a variable SESSION to the GLOBAL value or a value GLOBAL to the MySQL default value compiled-in one, then we apply the keyword DEFAULT.

But note that all system variables may not set to the value as DEFUALT and if it is proceeded to be used then MySQL results in error.

From the above example, the modifiers @@global and @@session and also @@ can apply only to the instant succeeding system variable but not any leftover system variables. View the below query statement that sets the global value of sort_buffer_size to 50000 and also the session value is set to 1000000:

SET @@global.sort_buffer_size = 50000, sort_buffer_size = 1000000;

Conclusion

Except, you have data specifying then, you need to escape subjectively growing the MySQL sort_buffer_size as well. The memory allocated here is also per connection. A user is instructed to avoid the increase in sort_buffer_size above the threshold of 2MB because it will slow down the allocation of memory hampering the performance penalty that can eradicate any assistance.

Recommended Articles

This is a guide to MySQL sort_buffer_size. Here we discuss the introduction, How does sort_buffer_size function work in MySQL with examples respectively. You may also have a look at the following articles to learn more –

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