Learn to use MySQL explain to execute the plan, and SQL performance tuning is no longer difficult

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The last article said MySQL Architecture system , come to know MySQL Server The optimizer at the end can generate Explain Implementation plan , The execution plan can help us analyze SQL Statement performance bottleneck , Optimize SQL Query logic , Study together today Explain Specific usage of execution plan .

1. explain Use

Use EXPLAIN Keyword can simulate optimizer execution SQL sentence , Analyze the performance bottleneck of your query statement or structure .
stay select Add... Before the statement explain keyword ,MySQL A tag will be set on the query , Execution query will return execution plan information , Will not implement this SQL.
Like the following one ：

What is the use of outputting so many columns ？

In fact, most of them are SQL Performance statistics of statements , First, briefly summarize the general function of each column , Let's talk about it in detail ：

2. explain Field details

Let's talk about the specific functions of each column in detail .

1. id Column

id Represents the sequence number of the query statement , Automatically assigned , The order is increasing , The bigger the value is. , The higher the execution priority .

id Phase at the same time , Priority is from top to bottom .

2. select_type Column

select_type Indicates the query type , Common are SIMPLE Simple query 、PRIMARY Main query 、SUBQUERY Subquery 、UNION The joint query 、UNION RESULT Joint provisional table results, etc .

3. table Column

table Express SQL Table name of statement query 、 Table alias 、 Temporary table name .

4. partitions Column

partitions Express SQL Query the partition that matches , If there is no partition NULL.

5. type Column

type Indicates the table connection type or data access type , Is how tables are connected , Or how to access the data .

The specific values are as follows , The order of performance from good to bad is ：

system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL

system

When there is only one row in the table , That is, system table , yes const Type of specials .

const

It means to use primary key or unique index for equivalent query , At most one record is returned . Good performance , Recommended .

eq_ref

Express Table joins The primary key or unique index is used , Below SQL It uses user Table primary key id.

ref

It means to use non unique index for equivalent query .

ref_or_null

It means to use non unique index for equivalent query , And it includes null Row of values .

index_merge

Represents the optimization logic using index consolidation , That is, multiple indexes used .

range

Indicates that the index range query is used .

index

Indicates that the index is used for full table scanning .

ALL

Indicates full table scan , The worst performance .

6. possible_keys Column

Indicates the index column that may be used , Actual queries may not necessarily be used .

7. key Column

Indicates that the index column is used in the actual query .

8. key_len Column

Indicates the number of bytes occupied by the index .

The number of bytes occupied by each type is as follows ：

type	Occupancy space
char(n)	n Bytes
varchar(n)	2 Bytes store variable length strings , If it is utf-8, Then the length 3n + 2
tinyint	1 Bytes
smallint	2 Bytes
int	4 Bytes
bigint	8 Bytes
date	3 Bytes
timestamp	4 Bytes
datetime	8 Bytes
Fields are allowed to be NULL	Additional increase 1 Bytes

9. ref Column

Express where Statement or table connection parameters compared with the index , Common are const（ Constant ）、func（ function ）、 Field name .

If no index is used , It is displayed as NULL.

10. rows Column

Indicates execution SQL The number of rows scanned by the statement .

11. filtered Column

Represents the percentage of table rows filtered by criteria .

Used to estimate the number of rows scanned when connecting with other tables ,row x filtered = 252004 x 10% = 25 Line ten thousand

12. Extra Column

Indicates some additional extended information , Not suitable for display in other columns , But it is very important .

Using where

Indicates that where Condition search , But no index is used .

Using index

Indicates that the overlay index is used , That is, the required data can be found on the index , There is no need to query the table twice , Good performance .

Using filesort

Indicates that external sorting is used , That is, the sorting field does not use the index .

Using temporary

Indicates that a temporary table is used , In the following example, a temporary table is used to store query results .

Using join buffer

Indicates when performing table Association , No index is used , The connection cache is used to store temporary results .

In the following example user_id There is no index in both tables .

Using index condition

Indicates use Index push down The optimization features of .

Summary of knowledge points ：

This article introduces in detail Explain Usage mode , And the meaning of each parameter . Whether it's a job or an interview , Use Explain Optimize SQL Inquire about , Are all necessary skills , Remember .

Let's learn the next part SQL Other optimization methods of query , Coming soon .

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