Beware of this common index failure scenario

In database optimization, indexes are the key to improving query performance. However, in some cases, query performance may not improve as expected even if we create indexes for the table. This is usually because the index fails in certain scenarios. This article will focus on an index failure scenario that is easy to fall into and provide corresponding solution strategies.

1. Index failure caused by implicit type conversion

When we perform implicit type conversion on index columns in query conditions, it is likely to cause index failure. This is because the database needs to perform type conversion on each row of data to match the query conditions when executing a query. This process cannot effectively utilize the index, resulting in a full table scan and reduced query performance.

Example

Suppose we have a user table userswith a user_idfield of type INTand we have created an index for this field.

CREATE TABLE users (
    user_id INT PRIMARY KEY,
    username VARCHAR(50)
);

CREATE INDEX idx_user_id ON users(user_id);
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If we execute the following query:

SELECT * FROM users WHERE user_id = '123'; -- 注意这里的'123'是字符串类型
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Although user_idthe field is of integer type, we used a string type value in the query criteria. This will cause the database user_idto perform implicit type conversion of the field to adapt to the string type query conditions, thus making the index invalid.

2. How to avoid index failure

1. Keep data types consistent : When writing query statements, ensure that the data types in the query conditions are consistent with the field types in the table. In the above example, we should use the query condition of integer type:

SELECT * FROM users WHERE user_id = 123; -- 使用整数类型
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2. Use the functions provided by the database for explicit type conversion : If you really need to perform type conversion, you can use the functions provided by the database for explicit conversion instead of relying on implicit conversion. Note, however, that explicit type conversions may also invalidate indexes, so they need to be used with caution.
3. Check and optimize query statements : Regularly check and optimize query statements to ensure that they are taking full advantage of the index. Using your database's query execution plan tool can help you identify which queries are not using indexes and find out why.
4. Reasonably design the database and index structure : When designing the database and index structure, fully consider the query requirements and data characteristics. Reasonable index design can improve query performance and reduce the possibility of index failure.

3. Conclusion

Indexes are a key tool for improving database query performance, but under certain circumstances, they can fail. Implicit type conversion is a common scenario that causes index failure. To make full use of indexes and improve query performance, we need to keep data types consistent, utilize explicit type conversions (if necessary), regularly check and optimize query statements, and properly design database and index structures. By following these best practices, we can avoid index staleness and thus improve the overall performance of our database.