MySQL uses the explain tool to view the execution plan

One 、Explain Tool is introduced

EXPLAIN Keyword can simulate optimizer execution SQL sentence , Analyze the estimated execution of your query statements .
stay SELECT Add... Before the statement explain keyword ,MySQL A tag will be set on the query , Execution query will return execution plan information , instead of
Implement this SQL.
Be careful ： If from Contains subqueries , The subquery will still be executed , Put the results in a temporary table .

Two 、Explain Field information in

2.1、id

id The number of the column is select The serial number of , Every id Corresponding to one select.id The higher the execution priority is ,id The same goes from top to bottom ,id by NULL Finally, execute .

2.2、 select_type

select_type Indicates the corresponding SQL Is the statement a simple or complex query ：

1. simple： Simple query , The query does not contain subqueries and union.
2. primary： The outermost layer of complex queries select.
3. subquery： Included in select Subqueries in .
4. derived： Included in from Subquery in Clause ,MySQL The results are stored in a temporary table , Also known as derived tables （derived table）
5. union： stay union The second and subsequent select

2.3、table

This column indicates explain Which table is being accessed by a row of .

1. When from When there are subqueries in clause ,table The column is <derivenN> Format , Indicates the current query dependency id=N Query for , So first execute id=N Query for .
2. When there is union when ,UNION RESULT Of table The value of the column <union1,2>,1 and 2 To participate in union Of select That's ok id.

2.4、type

This column represents the association type or access type , namely MySQL Decide how to look up rows in the table , Find the approximate range of data row records .
From the best to the worst are ：
system > const > eq_ref > ref > range > index > ALL
Generally speaking , We need to make sure that the query reaches range Level , It's best to achieve ref.

1. NULL：MySQL Be able to decompose query statements in optimization phase , You no longer need to access tables or indexes during execution . for example ： Select the minimum value... In the index column , can
   Complete with a separate lookup index , There is no need to access the table at execution time .
2. const：MySQL Can optimize some part of the query and convert it into a constant （ You can see show warnings Result ）. be used for primary key or unique key When all the columns of are compared with constants , So the table has at most one matching row , Read 1 Time , It's quite fast .
3. system：system yes const The special case of , When there is only one tuple in the table, it is system.
4. eq_ref：primary key or unique key All parts of the index are connected and used , At most, only one matching record will be returned . ordinary select This is not the case with queries type.
5. ref： comparison eq_ref, Don't use unique index , Instead, use the partial prefix of a normal index or a unique index , The index should be compared with a certain value , May be
   Multiple qualified rows found .
6. range： Range scan usually appears in in(), between ,> ,<, >= And so on . Use an index to retrieve a given range of rows .
7. index： Scan the full index to get the results , Generally scan a secondary index , This kind of scan does not start from the root node of the index tree to find quickly , Instead of directly
   Traverse and scan the leaf nodes of the secondary index , The speed is still relatively slow , This kind of query usually uses overlay index , The secondary index is generally small , So this
   Species usually ratio ALL faster .
8. ALL： That is, full scan , Scan all leaf nodes of your clustered index . In general, this requires adding indexes to optimize .

2.5、possible_keys

This column shows which indexes the query might use to find .
explain May appear possible_keys Listed , and key Show NULL The situation of , This is because there is not much data in the table ,MySQL Think index is not helpful for this query , Select full table query . If the column is NULL, There is no relevant index . under these circumstances , It can pass the inspection where Clause to see if you can create an appropriate index to improve query performance , And then use explain See the effect .

2.6、key Column

This column shows MySQL Which index is actually used to optimize access to the table .
If index is not used , Then the column is NULL. If you want to force MySQL To use or neglect possible_keys Index in column , Use in query force
index、ignore index.

2.7、key_len Column

key_len Column shows MySQL The number of bytes used in the index , With this value, you can figure out which columns in the index are specifically used .

key_len Calculation rules ：

1. String type ：
   char(n) and varchar(n),5.0.3 In later versions ,n Both represent the number of characters , Not the number of bytes , If it is utf-8, A number or letter takes up 1 Bytes , A Chinese character occupies 3 Bytes .
   char(n)： If you save Chinese characters, the length is 3n byte
   varchar(n)： If you save Chinese characters, the length is 3n + 2 byte , Plus 2 Bytes are used to store string length , because varchar Is a variable length string
2. value type
   tinyint：1 byte
   smallint：2 byte
   int：4 byte
   bigint：8 byte
3. Time type
   date：3 byte
   timestamp：4 byte
   datetime：8 byte

Be careful ：

1. If the field is allowed to be NULL, need 1 Is the byte record NULL
2. The maximum index length is 768 byte , When the string is too long ,MySQL Will do a similar left prefix index processing , Extract the first half of the characters and index them .

2.8、ref Column

ref The column shows in key In the index of the column record , The column or constant used to find the value of the table , Common are ：const、 Field name .

2.9、rows

rows The column is MySQL Estimate the number of rows to read and detect , Note that this is not the number of lines in the result set .

2.10、extra

This column shows additional information . Common important values are as follows ：

1. Using index： Use overlay index .
2. Using where： Use where Statement to process the result , And the columns of the query are not covered by the index .
3. Using index condition： The columns of the query are not completely covered by the index ,where In the condition is a leading range .
4. Using temporary：MySQL You need to create a temporary table to process the query . This situation is generally to be optimized , The first is to think of using rope
   Lead to optimization .
5. Using filesort： Will use external sort instead of index sort , Sort data from memory when data is small , Otherwise, you need to finish sorting on disk . In this case, we should also consider using index to optimize .
6. Select tables optimized away： When using some aggregate functions （ such as max、min） To access a field with an index , This result will appear .