Mysql execution plan analysis

 

 Mysql execution plan analysis:

　MySQL execution plan EXPLAIN can mainly be analyzed by type:

 

 

select_type represents the type of each select clause in the query (simple OR complex)

 

　　a.SIMPLE: The query does not contain subqueries or UNIONs
　　b. If the query contains any complex subsections, the outermost query is marked as: PRIMARY
　　c. A subquery is included in the SELECT or WHERE list, and the subquery is marked as: SUBQUERY
　　d. Subqueries contained in the FROM list are marked as: DERIVED
　　e. If the second SELECT appears after UNION, it is marked as UNION; if UNION is included in the subquery of the FROM clause, the outer SELECT will be marked as: DERIVED
　　f. The SELECT that gets the result from the UNION table is marked as: UNION RESULT

 

 

 

　　

 type indicates the way MySQL finds the desired row in the table, also known as "access type". The common types are as follows: (from top to bottom, the effect becomes better in turn)

 

　　ALL：Full Table Scan。 index：Full Index Scan。
　　range: Index range scan.
　　ref : Non-unique index scan.
　　eq_ref : Unique index scan.
　　const,system: Convert the query to a constant.
　　null: MySQL decomposes the statement during optimization and executes it without even accessing the table or index

 

All : full table scan, no index is used
range: index range scan, the scan of the index starts at a certain point, and returns rows that match the value range, commonly used in queries such as between, <, >, IN, etc.
ref: A non-unique index scan that returns all rows matching a single value.
eq_ref: Unique index scan, for each index key, only one record in the table matches it.
const, system: These types are used when MySQL optimizes a part of the query and converts it to a constant. If the primary key is placed in the where list, MySQL can convert the query into a constant.
　　system is a special case of const type. When the query table has only one row, use system.
NULL: MySQL decomposes the statement during optimization and executes it without even accessing the table or index.

 

 

 

如：explain    select * from account_balance where    userid='123'