[MySql] Explain notes

Explain

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Explain + SQL

Analysis of the implementation plan:

• id: This table represents the implementation of the priority

  • The same id, sequence table are sequentially performed from the top down;

     　              　　　　

• • Different id, and increments, id value is larger the higher execution priority, the first to be executed;

　　　　　　 

• select_type: a table reading operation of type

  • SIMPLE: Simple SELECT query, and the entire statement in the query does not contain <subquery> and <joint inquiry UNION>

  • PRIMARY: The most complex queries in the outer query, that is, PRIMARY

  • SUBQUERY: `` subquery

  • DERIVED: derived, with the rear FROM subqueries;  SELECT * from ( SELECT * from T1 WHERE ID = . 1 ) D1 

  • UNION: joint inquiry

  • UNION RESULT: UNION combined result set;

• table: This line represents the operation, which is a table of

• type : this entry representing the search query mode (7 species are the following descending order according to the Efficiency)

  • system: less than basic, it represents a table, only one record;

  • const:  the WHERE the above mentioned id = 1  specified constant inquiry;

  • eq_ref: unique index scan; an index key for the table, only one matching record; (such as id primary key index)

  • ref: non-unique index scan;  the WHERE name = ' ZhangSan '  , if ZhangSan more than one person, and the name field has an index, then the ref is retrieved;

  • Range:  WHERE ID BETWEEN 30 and 60  , or the like  WHERE ID in ( . 1 , . 3 , . 5 ) 

  • index:  SELECT id from User  where id is the primary key index;

  • ALL: to indicate a full table scan search method; ALL a genre search, needs to be optimized.

• possible_keys: possible use of the index

• Key : the index of actual use

• The number of bytes used in the index: key_len

　　　　

• ref: MySQL used to compare the value of the index value. Const single representation is constant comparison, if the name of a column, then compare apples to apples column.

• rows: a rough estimate of the number of rows per table is queried

• Extra : Additional information

  • Using filesort: Sort files, without being able to sort through the index, it will default file sorting; If this message appears, expressed the need for optimization;

  • Using temporary: the use of temporary tables, said in ordering, use the temporary table; also reminds us that this statement needs to be optimized;

  • Using index: indicates that the corresponding query using a covering index ;

    If accompanied by Using where: indicates that the index is used to perform a lookup index key values;

　　　　

　　　　　　If without Using where: indicates that the index is used to read the data, rather than seek operation;

　　　　

• • impossible where: indicating an error where, for example  WHERE name = ' Lisi ' and name = ' zhangsan ' 

  • distinct: automatic optimization distinct keywords, stops when it finds the first match

Explain examples

Execution order table:

1. 【select name, id from t2】

2. 【select id, name from t1 where other_column = ' '】子查询

3. [Select id, from t3] derived inquiry

4. [Select d1.name ...] outermost query

5. UNION operation