1. SQL prompt
use index is to give MySQL a hint to use the index as much as possible during execution. As for whether MySQL will accept the suggestion in the end, it is not certain.
ignore index is that mysql ignores an index.
force index forces mysql to use the index. mysql has no choice.
2. Covering index
Covering index and back-to-table query are two relative concepts. Let’s talk about back-to-table query first.
Back to the table: refers to go to the secondary index to search first, get the query result, get an id, and then load the data of this row in the clustered index according to the id. This process is called back-to-table.
