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In MySQL, a covering index is a special type of index that includes all the columns required by the query, not just the indexed columns themselves. When a query can be fully satisfied using the covering index, "MySQL can read data directly from the index without accessing the actual data row" , thereby improving query performance.
The benefit of using a covering index is reduced disk I/O and memory usage because MySQL does not need to load and process the actual data rows. This is especially useful for querying large tables or queries that need to be executed frequently.
To create a covering index, you need to ensure that the index contains all the columns required by the query. For example, if you have a table with columns A, B, and C, and your query only requires columns A and B, then you can create an index that includes columns A, B, and C so that the query is covered.
Here is an example showing how to create a covering index:
CREATE INDEX idx_covering ON your_table (column_a, column_b, column_c);
In this example, your_tableis your table name, column_a, column_band column_care the columns you want to include in the index.
Note that covering indexes are not suitable for all types of queries. "It is very efficient for queries that select a small number of columns, but may not be suitable for queries that need to return a large number of columns or perform complex calculations" . Also, having too many covering indexes can increase the overhead of write operations because the indexes need to be updated every time the table is updated.
Therefore, when designing an index, it is necessary to weigh the query performance and the overhead of writing operations, and decide whether to use a covering index according to specific query requirements.
This article is published by mdnice multi-platform