MySQL performance analysis - explain

Explain

explain+SQL sentence

What do you do

• 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

id*

select The serial number of the query , Contains a set of numbers , Represents execution in a query select The order of clauses or operation tables

Three situations

• id identical , Execution order from top to bottom
• id Different , If it's a subquery ,id The serial number is incremented ,id The higher the value, the higher the priority , The first to be executed
• id Same and different , At the same time .id If the same , It can be thought of as a group , From top to bottom ; In all groups ,id The bigger the value is. , The higher the priority , Execute first .

select_type

Common values ：

• SIMPLE ： ordinary select Inquire about , The query does not contain subqueries or UNION
• PRIMARY： If the query contains any complex Sub part , Outermost query marked
• SUBQUERY： stay select or where The list contains subqueries
• DERIVED： stay FROM The subqueries contained in the list are marked as derived ,MySQL These subqueries will be executed recursively , Put the results on the provisional list .
• UNION： If the second SELECT Appear in the UNION after , Is marked as UNION; if UNION Included in FROM Clause , Outer layer SELECT Will be marked as DERIVED
• UNION RESULT： from UNION Get the result in the table SELECT

Type of query , It is mainly used to distinguish ordinary queries , The joint query , Complex queries such as subqueries

table

Show which table this row's data is about

type*

• ALL： Read from the hard disk , Full table scan , The worst
• INDEX：index Type only traverses the index tree , Often than ALL fast , Because index files are usually smaller than data files
• 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 In the sentence between、<、>、in And so on , This range scan index scan is better than full table scan , Because it only needs to start with a certain point of the index , And end at another point , Don't scan all indexes .
• 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
• EQ_REF： Unique index scan , Build... For each index , Only one record in the table matches it , Common in primary key or unique index scanning
• CONST： Indicates that it is found through index once ,const For comparison 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 list ,MySQL You can convert the query to a constant
• SYSTEM： There is only one line in the table （ It's equal to the system table ）, This is a const Special case of type , Not usually
• NULL

From the best to the worst is system>const>eq_ref>ref>range>index>all

possible_keys

Show the indexes that may be applied to this table , One or more .

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

key*

Actual index used , If NULL, No index is used

If an overlay index is used in the query , The index appears only in key In the list

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 , Without losing accuracy , The shorter the length, the better .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

Shows which column of the index is used , If possible , It's a constant . Which columns or constants are used to find values on index columns .

rows*

According to table statistics and index selection , Roughly estimate the number of rows to read to find the required record

Extra*

Contains additional information that is not suitable for display in other columns but is important

• Using filesort ： explain mysql Will use an external index to sort the data , Instead of reading in the order of the indexes in the table .MySQL The sort operations that can't be done with indexes in are called “ File sorting ”
• Using temporary ： Use temporary table to save intermediate results ,MySQL Use temporary tables when sorting query results . It is common in sorting order by And group queries group by.
• USING index： It means corresponding select The override index is used in the operation (Covering Index), Avoid accessing the data rows of the table , Good efficiency ！ If it appears at the same time using where, Indicates that the index is used to perform index key value lookups ; If not at the same time using where, Indicates that the index is only used to read data and not to perform searches using the index . Overlay index (Covering Index)
• Using where： Indicates that where Filter
• using join buffer： Connection caching is used
• impossible where：where The value of the clause is always false, Can't be used to get any tuples
• select tables optimized away： In the absence of GROUPBY In the case of clause , Index based optimization MIN/MAX Operation or for MyISAM Storage engine optimization COUNT(*) operation , You don't have to wait until the execution phase to do the calculation , The query execution plan generation phase completes the optimization .