0730~Mysql optimization

Common reasons for index failure?

1. Use select*

2. There is no guarantee of the leftmost prefix principle

3. Do not add % to the left side of fuzzy query like -- solution covering index query

4.or improper use of keywords -- solution Use union to solve

5. There can be no calculation in the query condition

6. Functions cannot be used in query conditions

7.order by grouping order and set index are inconsistent, it will also fail

8.varchar type without single quotes

9. Use is null or is not null -- if the set property cannot be empty, then the index will not be taken,

If the set property can be empty, then is null will go to the index, is not null will not go to the index;

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When executing the plan with EXPLAIN, mainly check the following fields

type: The type of query, at least ref, All is the worst case;

key: see which indexes are actually hit;

rows: look at the number of query queries;

Extra: Mainly see UsingWhere means filter with where condition;