Detailed explanation of 9 common reasons for MySQL index failure

Catalog

Preface ：

1. The best left prefix rule

2. Primary key insertion order

3. Calculation 、 function 、 Type conversion ( Automatic or manual ) Cause index to fail

4. The column index to the right of the range condition fails

5. It's not equal to (!= perhaps <>) Cause index to fail

6.is null You can use index ,is not null Index not available

7.like Use wildcards % Invalid start index

8.OR As long as there are non indexed columns before and after , Will lead to index invalidation ?

9. Database and table character sets are used uniformly utf8mb4

Thank you very much ：

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Preface ：

MySQL One of the most effective ways to improve performance in is to design a reasonable index for the data table . Indexing provides an efficient way to access data , And speed up the query , Therefore, index has a crucial impact on the speed of query .

• Using an index, you can quickly locate a record in a table , So as to improve the speed of database query , Improve database performance .
• If the index is not used in the query , The query statement will scan all records in the table . In case of large amount of data , In this way, the query speed will be very slow .

Most of the time （ Default ） use B+ Trees To build the index . Only the index of spatial column type makes R- Trees , also MEMORY surface And support hash Indexes . Actually , Use without index Ultimately, the optimizer has the final say .

What optimizer is based on ？ be based on cost expenses (CostBaseOptimizer) , It's not based on rules （ Rule-BasedOptimizer）, It's not based on semantics . How to spend small, how to . in addition , SQL Whether the statement uses index , With the database version 、 Data volume 、 Data selection is all about .

**1.** The best left prefix rule

expand ：Alibaba《

Java Development Manual 》

Index file has B-Tree The leftmost prefix matching property of , If the value on the left is not determined , Then you can't use this index .

**2.** Primary key insertion order

If another primary key is inserted at this time, the value is 9 The record of , The insertion position is shown as follows ：

But this data page is full , What if you plug in again ？ We need to put the current Page splitting In two pages , Move some records in this page to the newly created page . What does page splitting and record shifting mean ？ signify ： Performance loss ！ So if we want to try Avoid such unnecessary performance loss , It's best to let the inserted record The primary key values are incremented , In this way, there will be no such performance loss .

So we suggest ： Let the primary key have AUTO_INCREMENT , Let the storage engine generate the primary key for the table itself ,

When inserting records, the storage engine will automatically fill in the self increasing primary key value for us . Such a primary key takes up less space , Write in sequence , Reduce page splits .

**3. Calculation 、 function 、 Type conversion ( Automatic or manual )** Cause index to fail

**4.** The column index to the right of the range condition fails

Example ：

# Create a federated index , Note the order of the fields

create index idx_age_classid_name on student(age,classid,name);

# Implementation plan

EXPLAIN SELECT SQL_NO_CACHE * FROM student

WHERE student .age = 30 AND student .classId > 20 AND student .name = ‘abc’ ;

# Create another federated index , Compared with the index above, the field order has changed

create index idx_age_name_classid on student(age,name,classid);

# Execute the same plan as like as two peas.

EXPLAIN SELECT SQL_NO_CACHE * FROM student

WHERE student .age = 30 AND student .classId > 20 AND student .name = ‘abc’ ;

You can see that both execution plans use indexes , however ：

• The first one is useless , Only the union index is used “idx_age_classid_name” Of age and classid.
• The second is to combine the index “idx_age_name_classid” Of age,name and classid It's all used .

5. It's not equal to (!= perhaps **<>)** Cause index to fail

6.is null**** You can use index ,is not null Index not available

**7.like Use wildcards %** Invalid start index

expand ：Alibaba《Java Development Manual 》

【 mandatory 】 Page search must not be left blurred or full blurred , If you need to go to search engine to solve .

8.OR As long as there are non indexed columns before and after , Will lead to index invalidation

**9.** Database and table character sets are used uniformly utf8mb4

Unified use utf8mb4( 5.5.3 Version above supports ) Better compatibility , The unified character set can avoid the chaos caused by character set conversion . Different Character set Before comparison transformation Will cause index invalidation .

Thank you very much ：

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