The timing of creating an index includes: 1. Query frequently uses a certain field as a filtering condition, especially when it can significantly reduce the result set, it is suitable to add single columns or composite indexes, but avoid fields with low distinction; 2. Fields that are often used for JOIN operations should be indexed, and the index order should be consistent with the JOIN order; 3. Indices can be used to avoid additional sorting for sorting, pay attention to index direction and order matching; 4. Overwriting index is suitable for scenes where all fields are included in the index, reducing back-to-table queries, and improving reading efficiency.
The key to creating an index in MySQL is whether you need to speed up certain queries. If you find that a query statement is executed slowly and often uses a certain field as a filter condition (such as WHERE
, JOIN
, or ORDER BY
), it may be the time to consider adding an index.

The following starts from several common scenarios and talks about when it is suitable to create an index.
1. Query frequently uses a certain field as filtering condition
If a field often appears in the WHERE
clause, especially when it can significantly reduce the number of result sets, you should consider adding an index to it. for example:

- When a user logs in, search for user information based on user name
- Filter data by order status in the order table
- Query records by time range in the log system
In this case, adding a single column index can significantly improve efficiency.
suggestion:

- Do not index fields with low distinction (such as gender fields)
- If it is a query combining multiple fields, you can consider composite indexing
- Be careful not to over-index, each index will affect write performance
2. Fields often used for connection (JOIN) operations
In multi-table association queries, if a field is often used as a join condition (such as a foreign key), then indexing this field will usually bring significant performance improvements.
For example:
SELECT * FROM orders JOIN customers ON orders.customer_id = customers.id;
If orders.customer_id
has no index, every JOIN may trigger a full table scan.
suggestion:
- Index the foreign key field (even if the foreign key is not explicitly defined)
- The index order should be consistent with the order of JOIN
- Composite indexes should consider query mode
3. Sort or group fields (ORDER BY / GROUP BY)
If you often sort or group a field, MySQL can use indexes to avoid additional filesorts, thereby speeding up response.
for example:
SELECT * FROM products ORDER BY price DESC LIMIT 10;
If the price
field has an index, this query can directly pass the index to complete the sorting without additional sorting operations.
Notice:
- Index direction (ASC/DESC) sometimes affects optimizer selection
- When sorting multiple fields, the index order must match the sort order.
- Grouping operations can also utilize indexes, but rely more on actual data distribution.
4. Covering Index usage scenarios
Overriding index means that all fields required by a query are included in the index. At this time, the database can directly obtain data from the index without back-to-table query, which is more efficient.
For example:
SELECT name, age FROM users WHERE department = 'IT';
If there is a compound index (department, name, age)
, then the query may hit the overwrite index.
benefit:
- Reduce disk I/O
- Improve query speed
- Especially suitable for read-only or read-only less read-only scenarios
Overall, the best time to create an index in MySQL is when you find that some queries are slower and the fields involved in these queries satisfy one of the several cases mentioned above. The more indexes, the better, but rather, it is about finding a balance between query performance and write overhead.
Basically that's it.
The above is the detailed content of when should i create an index in mysql. For more information, please follow other related articles on the PHP Chinese website!

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