Execution process of select statement in MySQL

MySQL The architecture of is shown in the figure below ：

One select The execution process of the statement is as follows ：

1. Establishing a connection ： By username and password and mysql Server establishes connection . If the username and password are incorrect , Is displayed “Access denied for user” A hint of .
2. The query cache ： Previously executed sql The result of the statement will be key-value The form of the pair is cached in memory .key yes sql sentence ,value Is the query result . If select Statements in key in , Then return directly value To the client . If you can't find... In the cache , Will continue to carry out the subsequent stages . Cache the results in memory after execution , For next query .( Cache invalidation is very frequent , for example update Then the cache will be emptied ).
3. Parser ： If no data is found in the cache , The parser will do sql Statement analysis . First pair sql Sentence for lexical analysis , analysis select * from table Compliance mysql Grammatical norms of . Then on sql Statement syntax analysis , If sql The statement is wrong , Is displayed You have an error in your SQL syntax Error warning .
4. Optimizer ： Before we start executing , The optimizer chooses which index to use based on the indexes in the table , Or is it join Select the order of table connection . So as to choose the best scheme .
5. actuator ： The executor will check the permissions of the current connection , The most direct way is to check whether the current connection is to the data table user Have query permission . If the current connection is to the data table user No query permission , An error with no permissions is returned . If the current connection has a connection to the data table user The query authority of , Will continue to execute . First, open the data table , At this point, the optimizer will be based on the storage engine used when creating the table , Use the interface of the corresponding storage engine to perform the query operation .