The explain statement in MySQL queries whether SQL is indexed, and several types in extra collate and summarize

1.using index Find through the secondary common index , Implement overlay index , There is no need to query the table

2.using index condition Find through the secondary common index , After the results found through the index, there are where filter , And this filtering can be realized only by using the secondary ordinary index , No judgment filtering in memory . But you need to query the required field values back to the table .

3.using where Whether or not it is found through the index , As long as the data is loaded into memory where Condition screening , All are

4.using index & using where： Search uses index , But all the data needed can be found in the index column , So you don't need to go back to the table to query the data

Practical example

If there is a watch xxx, to modify_date Index

1.using index Find through the secondary common index , Implement overlay index , There is no need to query the table

explain
SELECT modify_date
        from xxx

2.using index condition Find through the secondary common index , We need to go back to the table

explain
SELECT *
        from xxx where modify_date > "2022-06-27 10:27:07" and modify_date < "2022-06-28 0:0:0"

3.using where Search without index

explain
SELECT *
        from xxx
        where create_date > "2022-06-27 10:27:07" and create_date < "2022-06-28 0:0:0"

4.using index & using where： Search uses index , But all the data needed can be found in the index column , So you don't need to go back to the table to query the data

explain
SELECT modify_date
        from xxx where modify_date > "2022-06-27 10:27:07" and modify_date < "2022-06-28 0:0:0"

Return results