MySQL Index & execution plan

Catalog

One 、 What is index

Two 、 Classification of indexes

3、 ... and 、 Use cases of index creation

Four 、 Implementation plan

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One 、 What is index

1.1  What is index ？

An index is a combination of one or more columns in a database table , Its function is to improve the query speed of data in the table .
A catalogue similar to a book , Easy and fast positioning , Find the specified content .

1.2  Advantages and disadvantages of index ？

  advantage ： Speed up data query .
  shortcoming ： The time to create and maintain indexes has increased , At the same time, it takes up hard disk space .

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Two 、 Classification of indexes

2.1  Five indexes

   1) General index ： Is the most basic index , It has no restrictions ;

   2) unique index ： Value must be unique , But you can have an empty value . If it's a composite index , The combination of column values must be unique ;

   3) primary key :    A table can only have one primary key , No null values are allowed ;

   4) Composite index ： An index created on multiple fields , Only the first field that created the index is used in the query criteria , Indexes are used . When using a composite index, follow the leftmost prefix set ;

   5) Full-text index ;

2.2  modify & Delete index

        Modify the index

ALTER TABLE  Table name  ADD [UNIQUE|FULLTEXT] INDEX  Index name ( Field name [( length )][ASC|DESC]);

        Delete index

DROP INDEX  Index name  ON  Table name ;

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3、 ... and 、 Use cases of index creation

3.1  General index

Common query Single table ：

  Build index ：

--  Build index 
create index idx_moduleid on t_log(moduleid);

  Query again ： Speed up

 3.2  unique index ： The corresponding column value cannot be repeated

create unique index idx_url on t_log(url);

 3.3 primary key ： The fastest

3.4 Composite index ：

create index idx_userid_moduleid_url on t_log(userid,moduleid,url);

The leftmost segment matching principle ：

Four 、 Implementation plan

3.1 Execution plan syntax ：

MySQL tuning ：

# Example 1 ：explain

explain select * from t_user;

 

 # notes ： adopt left join and right join verification ;id equally （ Pay attention to the implementation of the plan table Column ）,left join Scan first a surface , Scan again b surface ;right join Scan first b surface , Scan again a surface

 

3.2 id The bigger it is , priority .

 3.3 select_type
#1）SIMPLE( Simple SELECT, Don't use UNION Or subquery, etc )

 

 3.4 PRIMARY( If the query contains any complex sub parts , The outermost select Marked as PRIMARY)
#   SUBQUERY( First in subquery SELECT)

 

3.5 Use index correctly

#1） Use like When the sentence is ,% The index is only used on the right .
# It works

# Invalid  

 #2）or The index will become invalid only if there is an unindexed column in the condition
# Invalid

 

#3） The type of condition is inconsistent
# Invalid

# It works

 #4）!= Number （ exception ： If it's a primary key , The index will go ）
# It works

order by

Be careful ：MySQL5.7 Invalid version

 

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