Explain usage and result analysis

1. What is? explain command

explain The command is to view MySQL The main methods of how the query optimizer executes queries , Can well analyze SQL Statement execution .

Whenever encountered slow execution SQL, You can use explain Order to check SQL Implementation of , And analyze according to the operation results , Adopt corresponding methods to SQL Statement optimization .

adopt explain We can get the following information ：

• Read order of tables
• Operation type of data read operation
• Which indexes can be used
• Which indexes are actually used
• References between tables
• How many rows per table are queried by the optimizer

2. How to use explain command

Just in SQL Use... Before the statement explain Key words can be used ;

Such as ：

EXPLAIN SELECT * FROM student_info WHERE student_id = 'A01234567';

3. analysis explain Command execution result

In execution explain After the command , You will get an output with many columns , Let's take a look at the meaning of each column .

3.1. id

1. If more lines id identical , The order of execution is from top to bottom ;
2. If it's a subquery ,id The serial number of will increase ,id The higher the value, the higher the priority , The first to be executed ;
3. If more lines id Some are the same and some are different , that id The same can be thought of as a group , Execute from top to bottom in the same group ;id Large groups are preferred ;

3.2. select_type

select_type What's shown is SELECT The type of ：

1. SIMPLE： ordinary SELECT, Not used UNION Or subquery ;
2. PRIMARY： Outermost layer SELECT;
3. UNION： The second floor , stay SELECT Then I used UNION;
4. DEPENDENT UNION：UNION The second in the statement SELECT, Rely on external subqueries ;
5. UNION RESULT：UNION Result ;
6. SUBQUERY： First in subquery SELECT;
7. DEPENDENT SUBQUERY： First in subquery SELECT, Depends on external queries ;
8. DERIVED： Export table's SELECT（FROM A subquery of a clause ）;

3.3. table

Displays which table the corresponding row is accessing , Usually the table name or the alias of the table （ If SQL It defines the alias ）;

3.4. type

type What is shown is what type of query is used , all type The order from good to bad is as follows ：
system > const > eq_ref > ref > range > index > all;

1. system： There is only one line in the table （ It's equal to the system table ）, This is a const Type of specials , Not usually , This can also be ignored ;
2. const： Indicates that it is found through index once ,const be used for primary key perhaps unique Indexes . Because only one line of data is matched , So soon . For example, place the primary key in where In the sentence ,MySQL You can convert the query to a constant ;
3. eq_ref： Unique index scan , For each index key , Only one record in the table matches it . Common in primary key or unique index scan ;
4. ref： Non unique index scan , Returns all rows that match a single value , In essence, it is also an index access , It returns all rows that match a single value , However , It may find more than one eligible row , So it should be a mixture of search and scan ;
5. range： Retrieve only rows in the given range , Use an index to select rows ,key Column shows which index is used , It's usually in your where Statement between, <, >, in And so on , This range scan index is better than full table scan , Because it just needs to start at a certain point in the index , And end at another point , Don't scan all indexes ;
6. index： Full table scan ,index And all The difference for index Type only traverses the index tree . This is usually better than all fast , Because index files are usually smaller than data files .（ That is to say, although all and index Read the whole watch , but index It's read from the index , and all It's read from the hard disk ） ;
7. all： Full table scan , Will traverse the entire table to find the matching rows ;

3.5. possible_keys

This column shows which indexes the query can use , This is based on the columns accessed by the query and the comparison operators used .

If there is an index on the field involved in the query , Then the index will be listed , But it doesn't have to be actually used by the query .

3.6. key

Show MySQL Decide which index to use to optimize access to the table . If NULL, No index is used .（ Possible causes include not indexing or index failure ）

If the index does not appear in possible_keys In the column , that MySQL It was chosen for other reasons, such as choosing an overlay index , Even if not where Clause .

In other words ,possible_keys It reveals which index is helpful for efficient row lookup , and key It shows which index the optimization adopts can minimize the query cost .

3.7. key_len

Shows MySQL The number of bytes used in the index , You can use this column to calculate the length of the index used in the query . Without losing accuracy , The shorter the length, the better .

3.8. ref

Show which field or constant is related to key Used together .

3.9. rows

Express MySQL Estimated number of rows to read in order to find the required row . According to the statistical information of the table and the selection of the index , This estimate may be very inaccurate . By putting all rows Multiply column values by , You can roughly estimate the number of rows that the entire query will check . The smaller the better. .

3.10. filtered

This column is MySQL 5.1 Newly added in , It shows that the table meets a certain condition （where Clause or join condition ） The percentage of a pessimistic estimate made by the number of records .

3.11. Extra

This column contains additional information that is not suitable for display in other columns . Common and most important values are ：

• Using index Express MySQL Will use overlay index , To avoid back table queries . Don't index overlay and index Access types are confused ;
• Using where Express MySQL The server will filter the rows after the storage engine retrieves them ;
• Using temporary Express MySQL A temporary table is used when sorting query results ;
• Using filesort Express MySQL An external index will be used to sort the results , Instead of reading rows from the table in index order .MySQL There are two file sorting algorithms , Both can be done in memory or on disk . but explain I won't tell you MySQL What kind of , It won't tell you whether the sorting is done in memory or on disk ;
• Range checked for each record（index map：N） Indicates that there is no useful index , The new index will be reevaluated on each row of the join .N Is shown in possible_keys Bitmap of index in column , And it's redundant ;
• Using union Express MySQL The index merging strategy is used , Index merging strategy is sometimes the result of optimization , But in fact, it often shows that the indexes on the table are badly built .