What is the function of the select... For UPDATE statement? Can you lock tables or rows?

1 Sentence meaning

In the project code , notice

select * from xxl_job_lock where lock_name = 'schedule_lock' for update

What does the above code mean

select Query statements are not locked , however select …for update In addition to the function of query , And lock it , And it's a pessimistic lock .

So is it a row lock or a table lock , It depends on whether the index is used / Primary key .

Useless index / A primary key is a table lock , Otherwise, it is a row lock .

2 Ideas

Create a new table , We use xxl-job The inside of the table

His watch , This field is the primary key

We first turn off automatic submission
Auto submit needs to be turned off , adopt set @@autocommit=0; Set to manual submit .0 On behalf of hand submit ,1 On behalf of auto submit .
It must be closed first , Otherwise, as soon as the statement is executed , Just submitted , We can't see the result we want

After the closing , Execute statement

select * from xxl_job_lock where lock_name = 'schedule_lock' for update

The above query statement means , Not only to query , And for this sql Statement to lock ; After locking , Other threads need to operate this table , Just got stuck , Wait until this sql Statement execution complete , Operations of other threads on this table , Will execute , Or I'll be waiting , In this way, the exclusive lock is realized

We can use select for update , It's an exclusive lock . To put it bluntly xxl-job Use a database table as a distributed lock , Make sure there are multiple xxl-job admin Under the node , Still, only one scheduling thread task can be executed at the same time

 Multithreading , Operate on the same database , We can use this database 
 Create a separate table , This table can be used as a distributed lock