What are the differences between using index and using where in executing extra in MySQL database

This article will explain in detail the differences between using index and using where in the implementation of extra in the MySQL database, and the content of the article is of high quality, so the editor will share it for you to do a reference. I hope you will have a certain understanding of the relevant knowledge after reading this article.

Using index

1, the column of the query is overwritten by the index, and the where filter condition is that the index is the leading column, and the Extra is Using index

Using where

1. The column of the query is not overwritten by the index, the where filter is not the leading column of the index, and the leading column in Extra is Using where.

2. The column of the query is not overwritten by the index, the where filter condition is not an index column, and Using where is used in Extra.

Using where means to filter where conditions by index or table scan. Conversely, there is no available index lookup. Of course, the cost of index scan + table return and table scan should also be considered here. The type here is all all, indicating that MySQL believes that full table scanning is a relatively low cost.

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