Introduction to the usage of explain and the meaning of result field in MySQL

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List of articles

• explain usage
• explain Introduction to the meaning of the result field
  • id
  • select_type
  • table
  • type
  • possible_keys
  • key
  • key_len
  • ref
  • rows
  • filtered
  • Extra

• explain Using examples
• summary
• Refer to the post
• Expanding reading

Use explain Query and analysis SQl The execution record of , Can be done sql Performance optimization ！

explain usage

mysql> explain select * from students;
+----+-------------+----------+------+---------------+------+---------+------+------+-------+
| id | select_type | table    | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+----------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | students | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
+----+-------------+----------+------+---------------+------+---------+------+------+-------+
1 row in set


mysql> explain extended select * from students;
+----+-------------+----------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table    | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+----------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | students | ALL  | NULL          | NULL | NULL    | NULL |    3 |      100 |       |
+----+-------------+----------+------+---------------+------+---------+------+------+----------+-------+
1 row in set

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explain Introduction to the meaning of the result field

id

• SELECT Identifier . This is a SELECT The query serial number of , Represents execution in a query select The order of clauses or operation tables ！

select_type

SELECT type , It can be any of the following :

• SIMPLE： Simple SELECT( Don't use UNION Or subquery )
• PRIMARY： The outermost SELECT
• UNION：UNION The second or later of SELECT sentence
• DEPENDENT UNION：UNION The second or later of SELECT sentence , Depends on external queries
• UNION RESULT：UNION Result
• SUBQUERY： First in subquery SELECT
• DEPENDENT SUBQUERY： First in subquery SELECT, Depends on external queries
• DERIVED： Export table's SELECT(FROM A subquery of a clause )

table

The table referenced by the output row ！

type

Connection type . Here are the various connection types , Sort from the best type to the worst type :

• system： The watch has only one line (= The system tables ). This is a const A special case of a join type .
• const： The table has at most one matching row , It will be read at the beginning of the query . Because there is only one line , The column values in this row can be considered constant by the rest of the optimizer .const The watch is fast , Because they only read once !
• eq_ref： For each row combination from the previous table , Read a row from the table . This is probably the best type of join , except const type .
• ref： For each row combination from the previous table , All rows with matching index values are read from this table .
• ref_or_null： The join type is like ref, But added MySQL You can search specifically for NULL Row of values .
• index_merge： The join type indicates that the index merge optimization method is used .
• unique_subquery： This type replaces the following form of IN The subquery ref: value IN (SELECT primary_key FROM single_table WHERE some_expr) unique_subquery Is an index lookup function , You can completely replace subqueries , More efficient .
• index_subquery： The join type is similar to unique_subquery. Can replace IN Subquery , But it is only suitable for non unique indexes in sub queries of the following forms : value IN (SELECT key_column FROM single_table WHERE some_expr)
• range： Retrieve only rows in the given range , Use an index to select rows .
• index： The connection type is the same as ALL identical , Except that only index trees are scanned . This is usually better than ALL fast , Because index files are usually smaller than data files .
• ALL： For each row combination from the previous table , Do a full table scan .

possible_keys

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

key

Show MySQL The index actually used in the query , If you don't use an index , Is shown as NULL

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 ）

ref

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

rows

Show MySQL The number of rows that it must check to execute the query . Multiplying the data between multiple rows can estimate the number of rows to process

filtered

Shows the percentage estimate of the number of rows filtered by the condition .

Extra

This column contains MySQL Solve query details

• Distinct：MySQL Find the first 1 After a match line , Stop searching for more rows for the current row combination .
• Not exists：MySQL Able to query LEFT JOIN Optimize , Find out 1 A match LEFT JOIN After the standard line , No longer check more rows in the table for the previous row combination . range checked for each record (index map: #):MySQL No good index can be found , But found that if the column value from the previous table is known , Maybe some indexes can be used .
• Using filesort：MySQL Need an extra pass , To find out how to retrieve rows in sort order .
• Using index： To retrieve column information in a table from an actual row that uses only the information in the index tree without further search .
• Using temporary： To solve the query ,MySQL You need to create a temporary table to hold the results .
• Using where:WHERE Clause is used to restrict which row matches the next table or is sent to the customer .
• Using sort_union(…), Using union(…), Using intersect(…): These functions show how to index_merge Join type merge index scan .
• Using index for group-by： Similar to accessing tables Using index The way ,Using index for group-by Express MySQL Found an index , It can be used to check Inquiry GROUP BY or DISTINCT All columns of the query , Don't search the hard disk extra to access the actual table .

explain Using examples

mysql> show keys from students;
+----------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table    | Non_unique | Key_name          | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+----------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| students |          0 | PRIMARY           |            1 | stud_id     | A         |           3 | NULL     | NULL   |      | BTREE      |         |               |
| students |          1 | index_create_date |            1 | create_date | A         |           3 | NULL     | NULL   | YES  | BTREE      |         |               |
+----------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set


mysql> explain  select * from students where stud_id = '1';
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table    | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | students | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |
+----+-------------+----------+-------+---------------+---------+---------+-------+------+-------+
1 row in set


mysql> explain  select * from students where create_date = '2010-01-01';
+----+-------------+----------+------+-------------------+-------------------+---------+-------+------+-------------+
| id | select_type | table    | type | possible_keys     | key               | key_len | ref   | rows | Extra       |
+----+-------------+----------+------+-------------------+-------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | students | ref  | index_create_date | index_create_date | 4       | const |    1 | Using where |
+----+-------------+----------+------+-------------------+-------------------+---------+-------+------+-------------+
1 row in set

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summary

• 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 .

Refer to the post

mysql in explain Meaning of usage and result

EXPLAIN Detailed command

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