MySQL Explain usage and parameter detailed explanation

The common performance bottlenecks of MySQL generally appear on the CPU and I/O, that is, when the data is loaded into the memory or read from the disk, the CPU is saturated or the loaded data is too large, and the memory capacity is insufficient.Disk I/O performance is limited.At this time, the Explain keyword is used for analysis and tuning.

• Basic usage

• Explain keyword can be used to simulate the optimizer to execute SQL query statement, so as to understand SQL statement processing method to achieve the performance bottleneck of analyzing query statement or table structure.

  Available through Explain

1. Table reading order and references
2. Basic types of data read operations
3. Available and actually used indexes
4. Number of query rows for the optimizer

• Parameter meaning

• id

Indicates the sequence number of the select query, that is, the order in which the select statement or operation table is executed in the query. The id obtained from the query has two cases, which can exist at the same time

The id is the same, the execution order is from top to bottom

The id is different. If it is a subquery, the serial number of the id will increase. The larger the id value, the higher the priority and the earlier it will be executed

select_type
The type of query, which is mainly used to distinguish different queries such as common query, union query, sub query, etc.There are several ways

1. SIMPLE: A simple SELECT query without subqueries or UNION

2. PRIMARY: When the query contains complex queries such as subqueries, the outer query is marked as PRIMARY

3. SUBQUERY: Include subqueries in SELECT or WHERE

4. DERIVED: Subquery included in FROM

5. UNION: query after the keyword UNION

6. UNION RESULT: Query to get results from UNION table

table
Indicates which table the data is queried from and which tables are associated

partitions
Indicates which partition data is searched for

type
Indicates which type is used in the query, the specific types are

system: The table has only one row of records, which is equivalent to the system table. This is a special case of the const type and basically does not appear.
Const: It means that the matching data is found once through the index, such as querying by the conditions matching the primary key
eq_ref: Unique index scan, there is only one corresponding record in each index key table, that is, the case of primary key or unique index and their corresponding data
ref: Non-unique index scan, that is, indexFind out the data corresponding to multiple conditions
range: Only retrieve rows in a given range, and use an index to select rows, usually range queries such as between, <,>, in, etc.
index: read from the indexFetch the whole table
all: read the whole table from the hard disk, that is, read the whole data
Query efficiency: rank from good to bad

possible_keys
Indicates the index that may be used, usually if there is an index on the query field, it will be displayed, althoughnot necessarily used

key
​​​​​​​The actual index used, if an overlay is used in the queryindex, the index will overlap with the select field of the query

key_len
​​​​​​​The length of the index, the shorter the better, the value is a tableCalculate the estimated maximum length instead of the actual used length

ref
​​​​​​​shows which column of the index is used

rows
​​​​​​​Estimated number of rows to read to find matching data

Extra
​​​​​​​Extra transactions are more important for analyzing retrieval efficiency, including the following:

Using filesort: MySQL uses an external index sort: "file sort", which means that the index order in the table cannot be used for reading
Using temporary: a temporary table is used, the information is usually used in sorting orAppears when grouping queries. MySQL uses temporary tables to store the query results that need to be sorted by order by and group by.Fetch data rows.Covering index: It is a new index based on the original index.
Using where: where filter is used. When using index appears at the same time, it means that the index is used to read the index key value, and the index key is the filter condition of where.
using join buffer: the connection cache is used.
impossible where: the value of the where clause is false and cannot be used to find data
select tables optimized away: Indicates that the index has optimized functions such as cont (*) in the query stage without group byOperation
distinct: Optimize the distinct operation, and stop searching for the same data after matching the first set of required data