Which SQL statements of mysql do not walk the index?

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It's not unfamiliar to mention indexes, but there are cases where indexes are not used in mysql, so let's take a look at the sql statements that don't take an index.

1. Index columns participate in expression evaluation.

SELECT 'sname' FROM' stu' WHERE 'age' + 10 = 30

2. Function operation.

SELECT 'sname' FROM' stu' WHERE LEFT ('date',4) < 1990

3.% words%-fuzzy query.

SELECT * FROM 'manong' WHERE `uname` LIKE' programmer%'- indexing SELECT * FROM 'manong' WHERE `uname` LIKE'% programmer%'- not indexing

4. Compare strings with numbers.

CREATE TABLE 'a' ('a'char (10)); EXPLAIN SELECT * FROM 'a'WHERE 'aindexes = "1"-- EXPLAIN SELECT * FROM 'a'WHERE 'aindexes 1-- not indexing, but also using functional operations

5. Or is included in the query condition.

Even if there is a conditional index, it will not be used. In other words, all fields required to be used must be indexed:

Select * from dept where dname='xxx' or loc='xx' or deptno = 45

6. Regular expressions do not use indexes.

The optimizer estimates that using a full table scan is faster than using an index and does not use an index.

The MySQL internal optimizer optimizes the SQL statement.

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