MySQL uses explain to analyze SQL execution plans to help find performance bottlenecks

When sql Execution time exceeded expectations （ It usually takes longer than expected ） when , Or want to know sql Execute whether to use the index of the design , Or hope to know the optimized sql Whether optimization processing is carried out as expected during execution , You can use explain Command view sql Implementation plan of .

The way to use it is in mysql Command the terminal to execute sql Add... Before the statement explain command .

mysql> explain select * from shard_bill where pay_status='0' and merchant_no='123' ;
+----+-------------+-------------------+------------+------+------------------------------+-------------+---------+-------+------+----------+-------------+
| id | select_type | table             | partitions | type | possible_keys                | key         | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------------------+------------+------+------------------------------+-------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | shard_bill | NULL       | ref  | merchant_no,merchant_no_date | merchant_no | 99      | const |    1 |       10 | Using where |
+----+-------------+-------------------+------------+------+------------------------------+-------------+---------+-------+------+----------+-------------+
1 row in set

The meaning of each column of data returned by the command execution is described below ： 

1. id

sql Sequence identification ,SQL From big to small execution .

• 1. id Phase at the same time , The order of execution is from top to bottom
• 2. If it's a subquery ,id The serial number of will increase ,id The higher the value, the higher the priority , The first to be executed
• 3.id If the same , It can be thought of as a group , From top to bottom ; In all groups ,id The bigger the value is. , The higher the priority , Execute first

2. select_type

Show each... In the query select The type of clause

• (1) SIMPLE( Simple SELECT, Don't use UNION Or subquery, etc )
• (2) PRIMARY( If the query contains any complex sub parts , The outermost select Marked as PRIMARY)
• (3) UNION(UNION The second or later of SELECT sentence )
• (4) DEPENDENT UNION(UNION The second or later of SELECT sentence , Independent of external queries )
• (5) UNION RESULT(UNION Result )
• (6) SUBQUERY( First in subquery SELECT)
• (7) DEPENDENT SUBQUERY( First in subquery SELECT, Independent of external queries )
• (8) DERIVED( Derived from table SELECT stay FROM A subquery of a clause )
• (9) UNCACHEABLE SUBQUERY( The result of a subquery cannot be cached , The first line of the outer link must be reevaluated )

3. table

• When the value is the table name , Indicates which table to query data from ;
• The value is the alias of the table , An alias is specified for the table when querying ;
• null Indicates that table query is not required ;

4. type

Common types are ：ALL, index,  range, ref, eq_ref, const, system, NULL( From left to right , Performance from poor to good ).

• ALL：Full Table Scan,MySQL Will traverse the entire table to find the matching rows , Scan the whole table row by row , Usually , Minimum efficiency ;
• index: Full Index Scan,index And ALL The difference for index Type only traverses the index tree , amount to data_all index Scan all inodes , amount to index_all;
• range: Retrieve only rows in the given range , Use an index to select rows , Can scan the range according to the index ;
• ref: Represents the join matching condition of the table , That is, which columns or constants are used to find values on index columns , Through the index column , It can be directly referenced to some data rows ;
• eq_ref: similar ref, The difference is that the index used is the only index , For each index key value , Only one record in the table matches , Simply speaking , It is used in multi table connection primary key perhaps unique key As a condition of Association ;
• const、system: When MySQL Optimize some part of the query , And convert to a constant , Use these types to access . For example, place the primary key in where In the list ,MySQL You can convert the query to a constant ,system yes const Special case of type , When the query table has only one row , Use system;
• NULL: MySQL Decompose statements during optimization , Execute without even accessing tables or indexes , For example, selecting the minimum value from an index column can be done through a separate index search ;
• const、system、NULL Refers to query optimization to constant level , You don't even need to find time .

5. possible_keys

The system estimates several possible indexes , But in the end, it can only be used 1 individual .

• Pointed out that MySQL Which index can be used to find records in the table , If there is an index on the field involved in the query , Then the index will be listed , But not necessarily used by queries ;
• This column is completely independent of EXPLAIN Output the order of the table shown . This means that possible_keys Some of the keys in cannot actually be used in the order of the generated tables .
• If the column is NULL, There is no relevant index . under these circumstances , It can pass the inspection WHERE Clause to see if it references certain columns or columns suitable for indexing to improve your query performance .

6. Key

Show MySQL The index actually decided to use .

If index is not selected , The key is NULL. To coercive MySQL To use or neglect possible_keys Index in column , Use in query FORCE INDEX、USE INDEX perhaps IGNORE INDEX.

7. key_len

Represents the number of bytes used in the index , You can use this column to calculate the length of the index used in the query (key_len The displayed value is the maximum possible length of the index field , It's not the actual length , namely key_len It is calculated according to the table definition , It is not retrieved from the table )

Without loss of accuracy , The shorter the length, the better ;

8. ref

Indicates the join matching condition of the above table , That is, which columns or constants are used to find values on index columns .

9. rows

MySQL According to table statistics and index selection , Estimated number of rows to read to find the required record ;

10. filtered

The row of the returned result accounts for the row to be read (rows The value of the column ) Percent of .

11. Extra

Performance from good to bad :useing index>usinh where > using temporary | using filesort

This column contains MySQL Solve query details , There are several situations ：

• Using temporary： Express MySQL You need to use temporary tables to store result sets , Common in sorting and grouping queries ;
• Using filesort：MySQL The sort operations that can't be done with indexes in are called “ File sorting ”;
• Using where： Column data is returned from a table that uses only the information in the index without reading the actual rows , This happens when all the request columns of the table are part of the same index , Express mysql The server will filter the rows after the storage engine retrieves them ;
• Using join buffer： This value emphasizes that the index is not used when getting the join condition , And you need to connect buffers to store intermediate results . If this value appears , That should pay attention to , Depending on the specific situation of the query, you may need to add indexes to improve the performance ;
• Impossible where： This value emphasizes where Statement will result in no eligible rows ;
• Select tables optimized away： This value means only by using the index , The optimizer may only return one line from the aggregate function result ;
• using  index Represents index coverage , That is, the column of the query is right in the index , There is no need to return to the physical line to query data .

12. EXPLAIN Deficiency

• EXPLAIN I won't tell you about triggers 、 The impact of stored procedure information or user-defined functions on queries ;
• EXPLAIN Don't think about all kinds of Cache;
• EXPLAIN Can't show MySQL Optimizations done when executing queries ;
• Some of the statistics are estimated , Not exactly ;
• EXPALIN Can only explain SELECT operation , Other operations need to be rewritten as SELECT Then check the execution plan .

13. Actually execute and view sql execution time

explain What is provided is only a general implementation plan , It is not the actual implementation effect , This is in most cases , Can reflect sql Whether the statement is insufficient . If you want to know sql The actual execution time , You can use the following method .

mysql> show variables like "profiling";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| profiling     | OFF   |
+---------------+-------+
1 row in set

mysql> set profiling = 1;
Query OK, 0 rows affected

mysql> show variables like "profiling";
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| profiling     | ON    |
+---------------+-------+
1 row in set

mysql> select * from boss_shard_bill_6 where bill_no='123' and pay_status='0' limit 10;
Empty set

mysql> show profiles;
+----------+-----------+------------------------------------------------------------------------------------+
| Query_ID | Duration  | Query                                                                              |
+----------+-----------+------------------------------------------------------------------------------------+
|        3 | 0.0017015 | select * from boss_shard_bill_6 where bill_no='123' and pay_status='0' limit 10    |
+----------+-----------+------------------------------------------------------------------------------------+
3 rows in set

mysql> set profiling = 1;
Query OK, 0 rows affected

By setting profiling Parameters , This parameter is usually set in the debugging or testing environment , Because this parameter decreases to a certain extent mysql Efficiency of execution , It is not suitable to enable this parameter in the production environment .