防止 MS-SQL 中的条件 INSERT/UPDATE 竞争条件

Question

我想知道我是否遵循了正确的方法，需要您的帮助才能弄清楚

这是我的非保护查询

DECLARE @cl_WordId bigint = NULL
SELECT
  @cl_WordId = cl_WordId
FROM tblWords
WHERE cl_Word = @cl_Word
AND cl_WordLangCode = @cl_WordLangCode
IF (@cl_WordId IS NULL)
BEGIN
  INSERT INTO tblWords (cl_Word, cl_WordLangCode, cl_SourceId)
    VALUES (@cl_Word, @cl_WordLangCode, @cl_SourceId)
  SET @cl_WordId = SCOPE_IDENTITY()
  SELECT
    @cl_WordId
END
ELSE
BEGIN
  SELECT
    @cl_WordId
END

为了保护它，我修改如下

DECLARE @cl_WordId bigint = NULL
SELECT
  @cl_WordId = cl_WordId
FROM tblWords WITH (HOLDLOCK)
WHERE cl_Word = @cl_Word
AND cl_WordLangCode = @cl_WordLangCode
BEGIN
  IF (@cl_WordId IS NULL)
  BEGIN
    INSERT INTO tblWords (cl_Word, cl_WordLangCode, cl_SourceId)
      VALUES (@cl_Word, @cl_WordLangCode, @cl_SourceId)
    SET @cl_WordId = SCOPE_IDENTITY()
    SELECT
      @cl_WordId
  END
  ELSE
  BEGIN
    SELECT
      @cl_WordId
  END
END

所以我在 select 查询中添加了 WITH (HOLDLOCK)，在 select 查询中添加了 begin 和 end

这种方法是否正确以防止条件INSERT/UPDATE竞争条件

Answer 1

正如我在您上一个问题 (Conditional INSERT/UPDATE Race Condition and “UPSERT” Race Condition With MERGE) 上发布的文章中提到的那样，使用 MERGE 和 HOLDLOCK 是线程安全的，因此您的查询将是：

MERGE tblWords WITH (HOLDLOCK) AS w
USING (VALUES (@cl_Word, @cl_WordLangCode, @cl_SourceId)) AS s (cl_Word, cl_WordLangCode, cl_SourceId)
    ON s.cl_Word = w.cl_Word
    AND s.cl_WordLangCode = w.cl_WordLangCode
WHEN NOT MATCHED THEN 
    INSERT (cl_Word, cl_WordLangCode, cl_SourceId)
    VALUES (s.cl_Word, s.cl_WordLangCode, s.cl_SourceId);

这看起来也可能是一个存储过程，您正在使用 SELECT @cl_WordId 到 return 调用者的 ID。这属于 Aaron Bertrand's bad habits to kick 之一，您应该使用输出参数，例如：

CREATE PROCEDURE dbo.SaveCLWord
        @cl_Word            VARCHAR(255), 
        @cl_WordLangCode    VARCHAR(255), 
        @cl_SourceId        INT,
        @cl_WordId          INT OUTPUT
AS
BEGIN

    MERGE tblWords WITH (HOLDLOCK) AS w
    USING (VALUES (@cl_Word, @cl_WordLangCode, @cl_SourceId)) AS s (cl_Word, cl_WordLangCode, cl_SourceId)
        ON s.cl_Word = w.cl_Word
        AND s.cl_WordLangCode = w.cl_WordLangCode
    WHEN NOT MATCHED THEN 
        INSERT (cl_Word, cl_WordLangCode, cl_SourceId)
        VALUES (s.cl_Word, s.cl_WordLangCode, s.cl_SourceId);

    SELECT  @cl_WordId = w.cl_WordId
    FROM    tblWords AS w
    WHERE   s.cl_Word = @cl_Word
    AND     s.cl_WordLangCode = @cl_WordLangCode;

END

ADDEDNUM

您可以在没有 MERGE 的情况下执行此操作，如下所示。

BEGIN TRAN

INSERT tblWords (cl_Word, cl_WordLangCode, cl_SourceId)
SELECT  @cl_Word, @cl_WordLangCode, @cl_SourceId
WHERE   NOT EXISTS
        (   SELECT  1
            FROM    tblWords WITH (UPDLOCK, HOLDLOCK)
            WHERE   cl_Word = @cl_Word
            AND     l_WordLangCode = @cl_WordLangCode
        );

COMMIT TRAN;

SELECT  @cl_WordId = w.cl_WordId
FROM    tblWords AS w
WHERE   s.cl_Word = @cl_Word
AND     s.cl_WordLangCode = @cl_WordLangCode;

如果你没有使用合并是因为你是 concerned about its bugs，或者因为在这种情况下你实际上没有做 UPDATE，所以 MERGE 是矫枉过正并且 INSERT 就足够了，那就够公平了。但是因为不熟悉语法而不使用它并不是最好的理由，花时间阅读它，了解更多信息，并在 SQL 弓上添加另一个字符串。

编辑

来自online docs

HOLDLOCK

Is equivalent to SERIALIZABLE. For more information, see SERIALIZABLE later in this topic. HOLDLOCK applies only to the table or view for which it is specified and only for the duration of the transaction defined by the statement that it is used in. HOLDLOCK cannot be used in a SELECT statement that includes the FOR BROWSE option.

因此在您的查询中，您有 6 个语句：

-- STATETMENT 1
DECLARE @cl_WordId bigint = NULL

--STATEMENT 2
SELECT
  @cl_WordId = cl_WordId
FROM tblWords WITH (HOLDLOCK)
WHERE cl_Word = @cl_Word
AND cl_WordLangCode = @cl_WordLangCode

BEGIN

--STATEMENT 3
  IF (@cl_WordId IS NULL)
  BEGIN

    -- STATEMENT 4
    INSERT INTO tblWords (cl_Word, cl_WordLangCode, cl_SourceId)
      VALUES (@cl_Word, @cl_WordLangCode, @cl_SourceId)
    SET @cl_WordId = SCOPE_IDENTITY()

    --STATEMENT 5
    SELECT
      @cl_WordId
  END
  ELSE
  BEGIN

    -- STATEMENT 6
    SELECT
      @cl_WordId
  END
END

由于您没有显式事务，每个语句都在其自己的隐式事务中运行，因此专注于语句 2，这等同于：

BEGIN TRAN

SELECT
  @cl_WordId = cl_WordId
FROM tblWords WITH (HOLDLOCK)
WHERE cl_Word = @cl_Word
AND cl_WordLangCode = @cl_WordLangCode

COMMIT TRAN

因此，由于 HOLDLOCK 在使用它的事务的持续时间内申请，锁被释放，此代码完成后锁被释放，所以当你进行到语句 3 和 4 另一个线程可能已插入到 table.

防止 MS-SQL 中的条件 INSERT/UPDATE 竞争条件

Preventing Conditional INSERT/UPDATE Race Condition in MS-SQL

sql

sql-server

conditional

sql-server-2016