Will mysql multi-condition query use index?

Mysql multi-condition query will use the index, which depends on the following factors: 1. The selectivity of the index, which refers to the proportion of unique or small repeated values ​​​​in the index; 2. The coverage of the index, which refers to the All columns required for querying, thus avoiding access to actual data rows and improving query performance; 3. Combine the query order and conditions, and optimize the strategy according to the specific conditions of the query.

Operating system for this tutorial: Windows 10 system, MySQL 8 version, Dell G3 computer.

In MySQL, multi-condition queries can use indexes to improve query performance.

Using indexes also depends on the following factors:

1. Selectivity of index:

   The index Selectivity refers to the proportion of unique or smaller duplicate values ​​in the index. If the columns in the query criteria are more selective, that is, if there are fewer duplicate values ​​in the data set, MySQL is more likely to use the index to execute the query.

2. Index coverage:

   MySQL may use a covering index if an appropriate index is created that includes the columns involved in the query. Execute the query. A covering index means that the index contains all the columns required for the query, thereby avoiding access to the actual data rows and improving query performance.

3. Combining the query order and conditions:

   If the conditions of the multi-condition query use AND connection, and the columns in the query conditions have With appropriate indexes, MySQL can effectively use these indexes to filter results. However, if there are OR joins or multiple different column conditions in the query, it may be more complicated, and MySQL will use an optimization strategy based on the specific conditions of the query when deciding whether to use an index.

It should be noted that if the data volume of the table is small, a full table scan may be faster than using an index. Additionally, too many indexes may increase the overhead of data modification, so this needs to be weighed against the cost of index creation and maintenance.

In summary, multi-condition queries can use indexes in MySQL to improve query performance, but whether to use indexes depends on the selectivity and coverage of the index as well as the structure and conditions of the query. Reasonable index design and query optimization can significantly improve the query efficiency of the database.

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