How Can I Execute SELECT Statements in MySQL Without Causing Locks?

Avoiding Locks with MySQL Queries

In MySQL, certain queries can cause locking, leading to performance issues especially when the table being queried is frequently modified. To address this, the question arises: is there a way to execute a select statement without inducing locks?

The original query in question:

SELECT COUNT(online.account_id) cnt from online;

locks the table due to the parallel event modifying the table as well. The question explores possible alternatives.

One potential solution mentioned in the responses involves setting the transaction isolation level to READ-UNCOMMITTED. However, this approach is not ideal for slave databases as it is incompatible with binary logging in STATEMENT mode.

An alternative approach that is compatible with slave databases is to use the following syntax:

SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED ;
SELECT * FROM TABLE_NAME ;
SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ ;

This method effectively disables locks for the duration of the select statement. However, it's important to note that this can lead to non-deterministic results.

Another suggestion is to use the following syntax:

SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED ;
SELECT * FROM TABLE_NAME ;
COMMIT ;

This option also disables locks for the duration of the select statement and ensures that changes made during the isolation period are committed immediately. It should be noted that this approach may have performance implications compared to the previous method. Additionally, it is not necessary to explicitly set the isolation level back to REPEATABLE READ as it will be automatically reset upon commit.

By employing these approaches, it is possible to avoid locks when performing select queries in MySQL, potentially improving performance and reducing contention. However, it is crucial to consider the potential trade-offs and implications for data accuracy and consistency when choosing the appropriate method.

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