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I am working on an IBM DB, but I think the concepts are pretty much the same.

I want to fetch a single line, return these data (or save them into variables let's say) and update some fields of that specific row. There might be a number of instance who try to perform that query in the same time, and therefore we need the operation of fetching to be atomic.

Each row has a field called pending, init to FALSE. When one instance fetches this row, this is set to TRUE (that's the update I want to do). Reason that I need it too be atomic, is that my query fetches the first row of the table, where pending is FALSE.

In pseudocode I have something like this :

OPEN_DB(myDb, "DBNAME");  // opening the DB
BEGIN_TRANSACTION(myDb);  // beginning transaction on my db
EXECUTE_QUERY(myDb,"SELECT * FROM tbname WHERE pending == 0 ORDER BY colid LIMIT 1");
... assign a cursor to my results
while (valid_data) {
    // assign column fields to variable

    // and here i want to do the update for this column
    // I guess the problem is here
     EXECUTE_QUERY(myDb,"UPDATE tbname SET pending = 1 WHERE colid=@colid")

}
COMMIT();

Don't worry about the syntax, they are macros and once executed on its own they work. I am guessing the problem is on the second query but why?

  • Should I commit first and then perform the second query?
  • If yes, then how can I ensure that there will be no other instance reading the same row before I update it?

I guess this doesn't have to do with the DB I am using.

If I comment the second one, code works. If I execute the second one stand alone, it works as well.

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1 回答 1

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选项 1 -为游标选择更新，这将是悲观锁定

选项 2 - 执行乐观锁并处理异常。锁需要一个列，如上次更新时间或版本，如下所示：

OPEN CURSOR

UPDATE table set col = :new_value
WHERE id = :id_from_cursor
last_updated = :last_updated_from_cursor;

CHECK if update row count == 1 COMMIT
IF not throw exception

尽管我认为选项 1 应该可以很好地工作，除非您在选择和更新之间有很大的时间间隔

于 2015-06-18T14:20:02.810 回答