MySQL index creation, optimization analysis and index optimization

One 、 Indexes

 Indexes ：  A sort data structure (Btree Indexes 、Hash Indexes 、full-text Full-text index 、R-Tree Indexes )
 effect ：  Assist in quick query 、 Update data of data table 

 Single value index ： An index contains only a single column , A table can have multiple single indexes 
 unique index ： The value of the index column must be unique , But you can have an empty value 
 Composite index ： An index contains multiple columns 

Two 、 Index Syntax

CREATE  [UNIQUE] INDEX index_name ON table_name (columname(length))   
ALTER TABLE tbl_ name ADD PRIMARY KEY (column_list): This statement adds a primary key , This means that the index value must be unique , And cannot be NULL.
ALTER TABLE tbl_ name ADD UNIQUE index_name (column_list): The value to create the index must be unique ( except NULL Outside ,NULL There may be many times )
ALTER TABLE tbl_ name ADD INDEX index_name (column_list): Add a normal index , Index values can appear multiple times .
ALTER TABLE tbL_ name ADD FULLTEXT index_name (column_list) The index specified is FULLTEXT, For full-text indexing .

3、 ... and 、 The index case

-  Fields that are frequently used as query criteria should be indexed 
-  Frequent new fields are not suitable for index creation ( Because not only are fields updated , And the new index )
- where You do not need to create an index for fields that are not used by the condition 
-  In the case of high concurrency, it tends to combine indexes 
-  Sort field in query , Will greatly improve the sorting speed 
-  Statistics or grouping fields in a query 

-  Table record too small 
-  Often add or delete tables （ Improve query speed , Reduced update speed     Not only save data , And the new index ）
-  Duplicate content of data  （ It doesn 't work ）

Four 、 Index optimization

• see SQL Statement execution plan , View the SQL Whether the statement uses an index , Did you do a full scan , Get the details of the access policy that may be considered by the optimizer ,

• ID

  • id identical , The execution sequence is from bottom to top (t2–>t3–>t1)
    

• id Different , If it is a subquery ,id The number will increase ;id The higher the value is, the higher the priority is, and the earlier the execution is (t3–>t1–>t2)
  

• - select_type

  • SIMPLE ordinary select Inquire about , The query does not contain subqueries or UNION
  • PRIMARY If the query contains any complex sub parts , The outermost query is marked PRIMARY
  • SUBQUERY stay SELECT or WHERE The list contains subqueries
  • DERIVED stay FROM The subqueries contained in the list are marked with DERIVED（ derivative ）,MySQL These subqueries will be executed recursively , Put the results in the provisional table
  • UNION If the second SELECT Appear in the UNION after , Is marked as UNION： if UNION Included in FROM Clause , Outer layer SELECT Will be marked as ：DERIVED
  • UNION RESULT from UNION Table to get the result SELECT

• type
  
  The best to worst order is ：system > const > eq_ref > ref > range > index > all
  all Full table scan Very violent and primitive search methods , It's time consuming and inefficient , There is a lot of room for optimization .

  • S There are 20000 people in the school , I'll tell you you can find Xiao Ming for me , Then what do you do ！ Of course, you are looking for 20000 people in the whole school one by one , Even if you are very lucky, the first person will find Xiao Ming , But you still can't stop , Because you can't confirm whether there is another Xiao Ming , Until you find 20000 people

  index Press In index order Full table scan
range Index scan with range , be relative to index Full index scan of , It has limits , So it's better than index
  ref_eq There is only one result set , Use primary key or unique index to search Precise query , No need to scan too much

  • For example, according to the student number to find a student in a school , Before we look it up, we know that there must be only one result , So when we first found this student number , The query stopped immediately

  const Put a primary key in where The following is used as a conditional query ,mysql The optimizer turns query optimization into a constant . As for how and when to transform , It depends on the optimizer

• possible_keys Available indexes , Not necessarily used when querying

• key Columns show MySQL Actual index used ; No index key is selected NULL

• key_len Length of index （key_len The displayed value is the maximum possible length of the index field , It's not the actual length ）
  Without loss of accuracy , The shorter the length, the better

• rows According to table statistics and index selection , Roughly estimate the number of rows to read to find the required record