MYSQL advanced explain

Introduction

1. Adding keywords before the sql statement explainwill display the execution information of sql instead of the result of sql execution, as follows

EXPLAIN SELECT * FROM student JOIN score ON student.`id` = score.`student_id`;

returns as follows

2. Let's introduce the meaning of the representatives in the query results one at a time

search result

id

• You can see the id as the order of execution. The larger the id, the higher the priority. When the id is the same, it will be executed from top to bottom
• If the id is empty, it will be executed sequentially from top to bottom

select_type

It is used to indicate whether a simple query or a complex query, and it has the following values

1. simple: simple query, excluding subqueries and union queries, as follows

EXPLAIN SELECT * FROM student WHERE id = 3;

2. primary: the outermost layer of the complex query select, the outermost query in SQL with subqueries is primary
3. union: the second select or the latter one of the union statement, the UNION RESULT is the merged result as follows

EXPLAIN SELECT * FROM student WHERE id = 3 
UNION 
SELECT * FROM student WHERE id = 4;

4. subquery: Indicates the subquery in select, not in the from statement

EXPLAIN SELECT * FROM student WHERE id = (SELECT student_id FROM score WHERE id=2);

5. DERIVED: The select of the derived table, the subquery behind from,

EXPLAIN SELECT * FROM (
	SELECT * FROM score WHERE id IN (1,3,5)
) t WHERE t.id =1;

Also appears as SIMPLE in some versions

table

• Indicates which table the data comes from
• Sometimes it is not the name of the real table, but a virtual table. The last digit of the virtual table is a number, which represents the query of the id

type

• Connection type, this is more difficult, and it is an important focus of our optimization, which directly reflects whether our SQL statement is efficient
• This field has many values, we mainly focus on these fields system, const, eq_ref, ref, range, index, all
  • The order of performance from good to check is: system>const>eq_ref>ref>range>index>all (important)

system

There is only one row in the table. This is a special case of the const type, which usually does not appear, so this field can be ignored

const

• Indicates that the index is found once, const is used to compare the primary key or unique index, because only one row of data is matched, so it is very fast
• primary keyIt should be noted that the or index is used in the query unique, so if a row of data is queried, it is not necessarily const

EXPLAIN SELECT * FROM attend_schedule_shift WHERE id = 12;

id is generally the primary key, so the type of query here is const

eq_ref

Unique index scan, only one record in the table is preset to match. Generally, two tables are associated, and the field in the association condition is a primary key or a unique index

EXPLAIN SELECT * FROM attend_schedule_shift 
JOIN attend_schedule_section 
ON attend_schedule_shift.id = attend_schedule_section.`sche_shift_id`;

The primary key id used in the above sql statement attend_schedule_shiftis associated with other tables, so attend_schedule_shiftthe query for this table is eq_ref

ref

• Non-unique index scan, returning all rows for a single value
• It is also an index access in essence. It returns all the rows that match a single value. It may find multiple eligible rows, so it should be a mixture of search and scan.

explain select * from time_attend_data where company_id = 420;

1. Composite index time_attend_dataThere is a composite index composed of company_id and date
2. Only one index value is used
3. This single value iscompany_id = 420

range

Essentially also an index lookup, this index must be a separate index

• Retrieves the given range of rows, using an index to select rows, and the key column shows which index to use.
• In the general conditional query, queries such as >, <, in, and between appear, but it is not necessarily

explain select * from time_attend_data where `id` between 300 and 600;

index

Traverse the index tree. Usually faster than ALL because index files are usually smaller than data files. Both all and index read the entire table, but index is retrieved from the index, while all is retrieved from the hard disk.

EXPLAIN SELECT `company_id`,`date` FROM time_attend_data;

Generally, the index is directly followed by select

ALL

• full table scan

possible_keys

• Displays indexes that may be applied to this table, but not necessarily actually used by the query. If the class is NULL, then there is no associated index.
• You can improve performance by checking where yourself to see if you can add an appropriate index

key

• The actual index to use.
• If possible_keysthere is a value, but the key is null, it may be that the data in the table is not much. Mysql thinks that it is not helpful for the current query and chooses the full table query.
• If you want to force the use of or ignore possible_keysthe index in mysql, then use when querying force index,ignore index

key_len

• Indicates the number of bytes used in the index, which can be used to calculate the length of the index used in the query.
• Generally speaking, the longer the index length, the higher the precision and low efficiency; the shorter the index length, high efficiency, but low precision.
• The length of the index is not really used, it is an estimated value

The maximum length of the index is 768 bytes. When the length is too large, mysql will perform a process similar to the leftmost prefix, and extract the first half of the characters for indexing. When the field is null, 1 byte is needed to record.

Calculation Rules

1. string
   1. char(n): n numbers or letters occupy n bytes, and Chinese characters occupy 3n bytes
   2. varchar(n): n numbers or letters occupy n bytes, and Chinese characters occupy 3n+2 bytes (+2 is used to store the string length)
2. number type
   1. tinyint: 1 byte
   2. smallint: 2 bytes
   3. int: 4 bytes
   4. bigint: 8 bytes
3. time type
   1. date: 3 bytes
   2. timestamp: 4 bytes
   3. datetime: 8 bytes

ref

Indicates which column is used, and the constant indicates that this column is equal to a certain constant.

rows

Roughly the number of rows that need to be read to find the desired record.

filter

Indicates the percentage of selected rows and read rows, 100 means 100% is selected, 80 means 80% is read

extra

Here are some, additional, more important information

Using filesort

• Using external index sorting instead of reading according to the index order in the table (generally needs to be optimized)
• The sorting that MYSQL cannot use the index to complete is called file sorting

Using temporary

Temporary tables are used to store intermediate results. Common in sorting order by and grouping query group by (best optimization)

using index

Indicates that in the select statement, the covering index is used, and the value is directly obtained from the index without reading data from the disk.

using where

where filter is used

using index condition

Newly added after 5.6, it means that the query column has an indexed column, and the index condition should be judged first