What is an index in MySQL? What kinds of indexes are commonly used? Under what circumstances will the index fail?

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Catalog

Preface

One 、 Definition of index

Two 、 Classification of indexes

  2.1 Single index

2.1.1 General index

2.1.2 unique index （unique）

2.1.3 primary key （primary key）

  2.2 Multi column index

2.2.1  Full-text index （fulltext）

3、 ... and 、 The advantages and disadvantages of index

  3.1 Advantages of index

  3.2  The disadvantages of indexing

Four 、 Application scenario of index

  4.1 Suitable for indexing

  4.2  Index failure

① The query criteria include or, Even if some conditions are indexed, they will fail .

② like Query to % start .

③ If the column type is string , Quotation marks are not used to reference data , Leading to no index .

④ Participate in the calculation on the index column .

⑤ Violate the leftmost matching principle .

⑥ When MySQL When full table scanning is faster than indexing .

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Preface

    In this age of big data , The huge amount of data requires efficient database operation technology , Index is an important object to improve the efficiency of database operation . We can understand it as “ Quickly find data structures in order ”.

    This article will start from the definition of index , Classification of indexes , The advantages and disadvantages of index , Share your understanding of the index in four aspects, such as the application scenario of the index , For your reference only .                                                                                             

Student surface ：

                                                        

One 、 Definition of index

        MySQL The official definition of ： Indexes (index) Help MySQL Data structure for efficient data acquisition .

         The essence ： An index is a data structure . It is a sequential table that describes the one-to-one correspondence between the column values of index columns and the record rows in the meta table .

        Conclusion ： In addition to the data itself , The database also maintains a data structure that satisfies a specific search algorithm , These data structures point to data in some way , In this way, the advanced search algorithm can be realized on the basis of these data structures , This data structure is the index . 

Be careful : 

① Index affects where Later sorting and finding （oder by）

② Indexes are often stored on disk in the form of files . 

Two 、 Classification of indexes

classification ：

Single index ： General index 、 unique index 、 primary key .

Multi column index ： Full-text index .

  2.1 Single index

        An index contains only a single column , But you can have multiple single-column indexes in a table .

        2.1.1 General index

                Definition ： The most basic index type , There is no limit to uniqueness .

--  Create a normal index 
CREATE INDEX  The index name  ON  Table name ( Field name );
CREATE INDEX stu_name on student(sname);
--  Modify the general index 
ALTER TABLE  Table name  ADD INDEX  The index name ( Field name );
ALTER TABLE student ADD INDEX sname(sname);

        2.1.2 unique index （unique）

                Definition ： Rows with the same index value are not allowed , So as to represent duplicate indexes and key values .

--  Add unique index 
CREATE UNIQUE INDEX  The index name  ON  Table name ( Field name );
CREATE UNIQUE INDEX stu_id ON student(sid);
--  Modify the unique index 
ALTER TABLE  Table name  ADD UNIQUE INDEX  The index name ( Field name );
ALTER TABLE student ADD UNIQUE INDEX sid(sid);

 

        2.1.3 primary key （primary key）

                Definition ： In the database diagram , Define a primary key , The primary key index will be created automatically .

--  Add primary key index 
--  When creating a table 
--  Modify the primary key index 
ALTER TABLE  Table name   ADD PRIMARY KEY ( Field name );
ALTER TABLE student ADD PRIMARY KEY (sid);

 

  2.2 Multi column index

        Definition ： An index created on a combination of fields in a table , Only when the left field of these fields is used in the query criteria , Indexes are used , Use a composite index to follow the leftmost prefix set .（）

       2.2.1  Full-text index （fulltext）

               Definition ： The key technology of index , Used to retrieve text information , In some words , Through the keywords or words in it , Find the record line to which it belongs .

--  Create full text index 
CREATE FULLTEXT INDEX  Full text index name  ON  Table name ( You want to set the field of full-text index ) WITH PARSER ngram;
CREATE FULLTEXT INDEX content ON student(ssex,sname) WITH PARSER ngram;
--  Modify the full-text index (with The latter representatives support Chinese )
ALTER TABLE  Table name  ADD FULLTEXT  Full text index name ( You want to set the field of full-text index ) WITH PARSER ngram;
ALTER TABLE student ADD FULLTEXT studentcontent(ssex,sname) WITH PARSER ngram;

  Full text index cannot be set for primary key ！！！

 

3、 ... and 、 The advantages and disadvantages of index

  3.1 Advantages of index

        ① Improve the efficiency of data retrieval , Reduce the IO cost .（ University Libraries index ）

        ② Sort data by index , Reduce sorting costs , Reduce CPU trumpet .

          nature ：

        （1） Efficiency ： Using index can improve the efficiency of database query .

        （2） integrity ： Users can accelerate the connection between meters , Achieve referential integrity between tables .

        （3） Uniqueness ： The index can ensure the uniqueness of the checked data .

        （4） Special abilities ： By using index , You can query in the process , Using the optimize hide tool , Improve system performance .

        * Find and sort faster .

  3.2  The disadvantages of indexing

        ① Creating and maintaining index groups takes time , And as the amount of data increases, so does the time spent .

        ② Index needs disk space , In addition to the data table taking up the data space , Each index takes up a certain amount of physical space . If there are a lot of indexes , Index files can reach the maximum file size faster than data files .

        ③ When adding data in the table 、 When deleting and modifying , Indexes should also be maintained dynamically , This reduces the speed of data maintenance .

        * Insert (insert)、 modify （update）、 Delete （delete） slower .

Four 、 Application scenario of index

  4.1 Suitable for indexing

         First of all ： On columns that often need to be searched , Can speed up the search ; 
         second ： On the column as the primary key , Force the uniqueness of the column and organize the data in the table ; 
         Third ： On columns that are often used in joins , These columns are mainly some foreign keys , Can speed up the connection ; 
         Fourth ： Create indexes on columns that often need to be searched by range , Because the index is sorted , Its specified range is continuous ; 
         The fifth ： Create indexes on columns that often need to be sorted , Because the index is sorted , In this way, the query can take advantage of index sorting , Speed up sorting query time ; 
         The sixth ： It is often used in WHERE Create an index on the column in clause , Speed up the judgment of conditions .

To make a long story short , The fields often used for searching are added with indexes .

  4.2  Index failure

    Use student form ,sid Set as primary key index ,sname Set to normal index .Navicat in type For query type ,ALL Retrieve for full table .

        ① The query criteria include or, Even if some conditions are indexed, they will fail .

for example ：

SELECT * FROM student WHERE Sid=1;

  There are or When

SELECT * FROM student WHERE Sid=1 OR birthday="1900-01-01";

 

        ② like Query to % start .

for example ：

SELECT * FROM student WHERE Sname=" Zhang San ";

  In the query statement like With % start

SELECT * FROM student WHERE Sname LIKE"% 3、 ... and ";

 

        ③ If the column type is string , Quotation marks are not used to reference data , Leading to no index .

for example ：

Did not hit the normal index , because 222 The string is not quoted .

SELECT * FROM student WHERE Sname=222;

 

        ④ Participate in the calculation on the index column .

for example ：

Missed sid primary key , Because of the calculation .

SELECT * FROM student WHERE Sid-1=1;

 

        ⑤ Violate the leftmost matching principle .

for example ：

because ssex The index is based on （ssex,sname） How to index the above , So for hit index .

SELECT * FROM student WHERE Ssex=" male ";

 

        ⑥ When MySQL When full table scanning is faster than indexing .

        ⑦ Other and upper index failure references Index failure is super detailed https://blog.csdn.net/sy_white/article/details/122112440?utm_source=app&app_version=5.3.0&code=app_1562916241&uLinkId=usr1mkqgl919blen

 

 

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