MySQL - SQL execution process

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One 、 Component is introduced

1、 The connector

1） Responsible for communication with clients , It's half duplex mode , This means that the client can only request from the server or the server can send data to the client at a fixed time , Not at the same time .
2） Verify that the user name and password are correct （ database mysql Of user Verify in the table ）, If an error occurs, an error notification is returned （deAcess nied for user ‘root’@‘localhost’（using password：YES））, If correct , Will go mysql Permission table for （mysql Medium user、db、columns_priv、Host surface , The global level is stored separately 、 Database level 、 Table level 、 Column level 、 coordination db Database level for ） Query the permissions of the current user .

2、 cache （Cache）

Also known as query caching , The stored data is stored in the form of key value pairs , If caching is turned on , So in a query sql Statement will first determine whether the cache contains the current sql Statement key value pairs , If there is, return the corresponding result directly , If it doesn't exist, perform the next series of operations . If it is not turned on, skip it directly .

3、 analyzer

From the client sql Analyze , This will include preprocessing and parsing , And the keyword extraction 、 analysis , And make up a parse tree . Specific analytic words include but are not limited to select/update/delete/or/in/where/group by/having/count/limit etc. , If we analyze grammatical errors , Will directly throw the client exception ：ERROR:You have an error in your SQL syntax.
such as ：select * from user where userId =1234;
In the parser, we use the semantic rules to select from where These keywords are extracted and matched ,mysql Will automatically determine keywords and non keywords , Identify the user's matching fields and custom statements . There will also be some checks at this stage : For example, check whether the current database exists user surface , At the same time, if User There is no userId This field will also report an error ： unknown column in field list.

4、 Optimizer

Enter the optimizer description sql The statement conforms to the standard semantic rules and can be executed . The optimizer selects the best choice based on the execution plan , Match the appropriate index , Choose the best solution . For example, a typical example is ：
surface T, Yes A、B、C Columns are federated (A,B,C), During the query , When sql The query condition is :select xx where B=x and A=x and C=x. A lot of people think it's useless to use indexes , But it actually uses , Although the index must conform to the leftmost principle to be used , But in essence , The optimizer will automatically send this sql Optimize to :where A=x and B=x and C=X, This optimization will be for the underlying to match the index , At the same time, in this stage, preprocessing is performed automatically according to the execution plan ,mysql Accounting calculates the best time for each method of execution , To finalize an executive sql To the last actuator .
The optimizer will be based on Number of scanning lines 、 Whether to use temporary table 、 Whether to sort, etc. to determine whether to use an index , The number of scanning lines can be estimated through statistical information , Statistics can be regarded as the number of unique indexes , Partial sampling can be used to estimate , Specifically, choice N Data pages , Count the different values of the data on these pages , Get an average of , Then multiply by the number of pages in this index , Got it. . But because the index data will change , So the statistics of the index will also change . When the number of data lines changed exceeds 1/M When , The statistics will be recalculated .

5、 actuator

The executor will call the corresponding storage engine to execute sql. The mainstream is MyISAM and Innodb .
MyISAM and Innodb The index data structure is B+ Trees
memory The index data structure is hash surface

Two 、 Execution process

1、 Write operations

2、 Read operations

The process of query is similar to that of update , But it's a little different , This is mainly due to the differences in their search and filtering , Update because it will be updated after searching , So the query is always in the buffer pool （ The data page that uses the index and contains the data in the buffer pool ）. And the query is more complex .
Create table ：

create table tbl_test (a int primary key, b int, c int, d int, e varchar(50));create index idx_bcd on tbl_test(b, c, d);insert into tbl_test values (4,3,1,1,'a');insert into tbl_test values (1,1,1,2,'d');insert into tbl_test values (8,8,7,8,'h');insert into tbl_test values (2,2,1,2,'g');insert into tbl_test values (5,2,2,5,'e');insert into tbl_test values (3,3,2,1,'c');insert into tbl_test values (7,4,0,5,'b');insert into tbl_test values (6,5,2,4,'f');

In execution select * from tbl_test where b >= 2 and b < 7 and c > 0 and d != 2 and e != ‘a’; At the time of extraction , Will Where The condition is split into Index Key（First Key & Last Key）、Index Filter And Table Filter .
1、Index Key
Used to determine the SQL The sequential range of queries in the index （ The starting point + Termination point ） Query criteria for , It's called Index Key; Because of a range , It contains at least one starting condition and one ending condition , therefore Index Key Also split into Index First Key and Index Last Key , Respectively used to locate the starting point of index search to the ending point .
Index First Key
Used to determine the starting point of index query range ; Extract rules ： Start with the first key value of the index , Check that it's in where Is there... In the condition , If it exists and the condition is =、>= , Then add the corresponding conditions to Index First Key In , Continue reading the next key value of the index , Use the same extraction rules ; If it exists and the condition is > , Then add the corresponding conditions to Index First Key in , To terminate at the same time Index First Key Extraction of ; If it does not exist , Also terminate Index First Key Extraction of .
in the light of SQL：select * from tbl_test where b >= 2 and b < 7 and c > 0 and d != 2 and e != ‘a’, Apply this extraction rule , extracted Index First Key by b >= 2, c > 0 , because c On the condition that >, End of extraction
​ Index Last Key
The endpoint used to determine the scope of the index query , And Index First Key Just the opposite ; Extract rules ： Start with the first key value of the index , Check that it's in where Is there... In the condition , If it exists and the condition is =、<= , Then add the corresponding condition to Index Last Key in , Continue to extract the next key value of the index , Use the same extraction rules ; If it exists and the condition is < , Then add the condition to Index Last Key in , At the same time terminate extraction ; If it does not exist , Also terminate Index Last Key Extraction of
in the light of SQL：select * from tbl_test where b >= 2 and b < 7 and c > 0 and d != 2 and e != ‘a’, Apply this extraction rule , extracted Index Last Key by b < 7 , Because it is < Symbol , End of extraction
2、Index Filter
At the completion of Index Key After the extraction of , We according to the where Conditions fix the index's query range , So is it that every index item in the range satisfies WHERE The conditions are ？ Obviously 4 , 0 , 5 , 2 , 1 , 2 All belong to the scope , But they are not satisfied SQL Query criteria for
therefore Index Filter Used after index range determination , determine SQL What other conditions in can be filtered using indexes ; Extract rules ： Start with the first column of the index column , Check that it's in where Is there... In the condition , If it exists and where The condition is only =, Then skip the first column and continue to check the index next column , The next index column takes the same extraction rules as the first one ; if where Condition is >=、>、<、<= Some of them , Skip the first column of the index , Put the rest where All index related columns in the condition are added to Index Filter In ; If the first column of the index where Conditions include =、>=、>、<、<= Other conditions , Then this condition and the rest where All index related columns in the condition are added to Index Filter In ; If the first column does not contain query criteria , All index related conditions are added to Index Filter In
in the light of SQL：select * from tbl_test where b >= 2 and b < 7 and c > 0 and d != 2 and e != ‘a’, Apply this extraction rule , extracted Index Filter by c > 0 and d != 2 , Because the first column of the index only contains >=、< Two conditions , So the first column skips , Put the rest of c、d Add two columns to Index Filter in , End of extraction
3、Table Filter
This is a little bit easier ,where All the conditions that can't be filtered by index are included in this ; Extract rules ： All queries that do not belong to index columns , All belong to Table Filter In
in the light of SQL：select * from tbl_test where b >= 2 and b < 7 and c > 0 and d != 2 and e != ‘a’, Apply this extraction rule , that Table Filter for e != ‘a’
stay 5.6 Before , It doesn't matter Table Filter And Index Filter Of , These two conditions are directly assigned to Server Layer to filter . The screening process is based on Index Key The conditions of the engine layer are initially screened , Then get the corresponding primary key value, and query back to the table to get the row records initially screened , Pass in Server Layer for subsequent filtering , stay Server Because the index is not used in layer filtering, the whole table scan will be performed . And the optimization of index push down is to make Index Filter Push to the engine layer , In the use of Index First Key And Index Last Key When screening , Take it Index Filter The conditions are screened again , In this way, we can filter out the primary key values corresponding to unqualified records , Reduce the number of times to return to the table , At the same time Server There will also be fewer layers to record , The efficiency of full table scanning and filtering will also be improved .
Here is not used Schematic diagram of index push down and using index push down ：

Here is Use Schematic diagram of index push down and using index push down ：

3、 summary

Note here that if you don't use the index in the beginning , The data pages on the disk will be read to the buffer pool for query in turn .

3、 ... and 、 summary

Original address ：https://www.cnblogs.com/mengxinJ/p/14045520.html#_label2
author ： Fried cat

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