Question

I have table

tasks
id, started_at, ...
1, 2017-01-01 10:00:00
2, 2017-01-01 10:00:00 
3, NULL
4, NULL
...

SELECT * FROM TASKS WHERE started_at IS NULL LIMIT 10

save ids of above query to ids variable

UPDATE tasks SET started_at = <<NOW>> WHERE id IN <<ids>>

Now I need to make sure that no other process reads the same ids before the update has happened.

I tried:

START TRANSACTION;
SELECT * FROM tasks WHERE requested_at IS NULL LOCK IN SHARE MODE;

and

START TRANSACTION;
SELECT * FROM tasks WHERE requested_at IS NULL FOR UPDATE;

However I still got the same resultset when querying from another connection even though I have not finished the transaction.

Answer 1

我认为UPDATE是应该发生锁定的地方，因为这是您不希望其他人查询并找回不完整/不一致数据的点。因此，您可以尝试执行UPDATE，如下所示：

LOCK TABLES tasks WRITE;
    UPDATE tasks SET started_at = <<NOW>> WHERE id IN <<ids>>
UNLOCK TABLES;

另一种选择是在事务中执行更新：

START TRANSACTION;
    UPDATE tasks SET started_at = <<NOW>> WHERE id IN <<ids>>
COMMIT;

在任何一种情况下，在UPDATE完成之前，其他任何人都无法访问正在更新的记录。