How does MySQL prepare SQL (solve the problem that in query SQL preprocessing can only query one record)

One 、 Preface / summary

My colleague asked me a question this week ,SQL How to use placeholders in ? Preprocessing , Why did he use prepare It didn't work ;

1、prepare brief introduction

Execute one more time SQL When the sentence is , If you deal with this every time SQL sentence , Generate execution plan , It is bound to waste a certain amount of time .

SQL Preprocessing （Prepare）, It's a special kind SQL Processing mode ; Preprocessing will not be performed directly SQL sentence , But first SQL Statement compilation , Generate execution plan , And then through Execute Order to carry SQL Parameter execution SQL sentence .

• Prepare Is widely used , most ORM There are frames API Support ;
• Prepare Can improve SQL Executive performance , It can also prevent SQL Security problems caused by injection ;
• Prepare Although the syntax varies greatly in each database , But usually we don't write by hand SQL, But use ORM Frame to do ;

2、prepare The origin of ？

from mysql Server execution sql In terms of the process ,SQL The execution process includes the following stages Lexical analysis -> Syntax analysis -> Semantic analysis -> Execute plan optimization -> perform .

Lexical analysis -> The two stages of parsing are called hard parsing .

• Lexical analysis and recognition sql Every word in ;
• Parsing SQL Whether the statement conforms to sql grammar , And get a grammar tree （Lex）.

For, only the parameters are different , Others are the same sql, Their execution time is different, but the hard parsing time is the same .

And the same SQL As the query data changes , The execution time of multiple queries may be different , But the time of hard parsing is constant . So for sql Short execution time ,sql The higher the ratio of hard parsing time to total execution time .

Prepare The emergence of is to optimize the problem of hard parsing .

although Prepare stay execute Phase can save hard parsing time . But if sql Only once , And in prepare By , that sql The execution needs to interact with the server twice （Prepare and execute）, And ordinary （ Not prepare） The way , Only one interaction is needed . Use this way prepare It will bring additional network overhead , Do more harm than good .

If the same sql It needs to be done many times , such as ： With prepare How to execute 10 Time , Then only one hard parsing is needed . Now The extra network overhead is minimal ; therefore prepare It is more suitable for frequently executed SQL in .

Two 、prepare do IN Inquire about

MySQL Pretreatment is a group SQL Set of operations , It has no fixed syntax format , But in most cases, it will follow the following 3 Use... In two steps ：

1. Use PREPARE Instructions are predefined SQL Statement template ;
2. Use SET Instruction definition SQL Parameters ;
3. Use EXECUTE Instructions carry parameters to execute SQL Templates .

1、 Preprocessing IN The query failed ！

SQL：

prepare myFun from 'select * from user where id IN (?)';
set @str='1,2';
execute myFun using @str;

Above SQL Can do three things ：

1. Create preprocessing SQL（myFun）, Where you want to use variables use ? Express ;
2. Set a variable @str;
3. Perform preprocessing SQL（myFun）,USING Followed by parameters , If there are multiple parameters （ That is, multiple placeholders ?） Use English commas , Separate ;

SQL Execution results ：

from SQL As a result of implementation , Find preprocessing SQL Not fully effective , Only one row of records was found ; Normally, two lines of records should be found ;

2、 Solve pretreatment IN Query failure

Use as follows SQL Replace ：

prepare myFun from 'select * from user where FIND_IN_SET(id,?)';
set @str='1,2';
execute myFun using @str;

SQL Execution results （ In line with expectations ）：

3、MyBatis and JDBC Dynamic in IN Inquire about

mybatis Support in The dynamics of the sql Inquire about , Its input parameter can be a Collection, The bottom layer will Collection The content of is constructed using English commas , Delimited string ; It can also be an English comma , Delimited string .

And the original jdbc in sql I won't support it IN Directly for a single placeholder ? Pass in a comma concatenated string , for example in (‘001’, ‘002’), Developers need to build placeholders according to the number of parameters passed in ? The number of ;

For example, pass in an array or List,{‘001’, ‘002’}; Need to construct in by ：in(?, ?);
And then through PreparedStatement#setString() Methods build dynamically SQL sentence ,.