Posted By: Anonymous
While researching the use of Table Hints, I came across these two questions:
Answers to both questions say that when using
(UPDLOCK, HOLDLOCK), other processes will not be able to read data on that table, but I didn’t see this. To test, I created a table and started up two SSMS windows. From the first window, I ran a transaction that selected from the table using various table hints. While the transaction was running, from the second window I ran various statements to see which would be blocked.
The test table:
CREATE TABLE [dbo].[Test]( [ID] [int] IDENTITY(1,1) NOT NULL, [Value] [nvarchar](50) NULL, CONSTRAINT [PK_Test] PRIMARY KEY CLUSTERED ( [ID] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] ) ON [PRIMARY]
From SSMS Window 1:
BEGIN TRANSACTION SELECT * FROM dbo.Test WITH (UPDLOCK, HOLDLOCK) WAITFOR DELAY '00:00:10' COMMIT TRANSACTION
From SSMS Window 2 (ran one of the following):
SELECT * FROM dbo.Test INSERT dbo.Test(Value) VALUES ('bar') UPDATE dbo.Test SET Value = 'baz' WHERE Value = 'bar' DELETE dbo.Test WHERE Value= 'baz'
Effect of different table hints on statements run in Window 2:
(UPDLOCK) (HOLDLOCK) (UPDLOCK, HOLDLOCK) (TABLOCKX) --------------------------------------------------------------------------- SELECT not blocked not blocked not blocked blocked INSERT not blocked blocked blocked blocked UPDATE blocked blocked blocked blocked DELETE blocked blocked blocked blocked
Did I misunderstand the answers given in those questions, or make a mistake in my testing? If not, why would you use
(UPDLOCK, HOLDLOCK) vs.
Further explanation of what I am trying to accomplish:
I would like to select rows from a table and prevent the data in that table from being modified while I am processing it. I am not modifying that data, and would like to allow reads to occur.
This answer clearly says that
(UPDLOCK, HOLDLOCK) will block reads (not what I want). The comments on this answer imply that it is
HOLDLOCK that prevents reads. To try and better understand the effects of the table hints and see if
UPDLOCK alone would do what I wanted, I did the above experiment and got results that contradict those answers.
Currently, I believe that
(HOLDLOCK) is what I should use, but I am concerned that I may have made a mistake or overlooked something that will come back to bite me in the future, hence this question.
Why would UPDLOCK block selects? The Lock Compatibility Matrix clearly shows
N for the S/U and U/S contention, as in No Conflict.
As for the HOLDLOCK hint the documentation states:
HOLDLOCK: Is equivalent to SERIALIZABLE. For more information, see SERIALIZABLE
later in this topic.
SERIALIZABLE: … The scan is performed with the same semantics as a transaction running at the SERIALIZABLE isolation level…
and the Transaction Isolation Level topic explains what SERIALIZABLE means:
No other transactions can modify data that has been read by the
current transaction until the current transaction completes.
Other transactions cannot insert new rows with key values that would
fall in the range of keys read by any statements in the current
transaction until the current transaction completes.
Therefore the behavior you see is perfectly explained by the product documentation:
- UPDLOCK does not block concurrent SELECT nor INSERT, but blocks any UPDATE or DELETE of the rows selected by T1
- HOLDLOCK means SERALIZABLE and therefore allows SELECTS, but blocks UPDATE and DELETES of the rows selected by T1, as well as any INSERT in the range selected by T1 (which is the entire table, therefore any insert).
- (UPDLOCK, HOLDLOCK): your experiment does not show what would block in addition to the case above, namely another transaction with UPDLOCK in T2:
SELECT * FROM dbo.Test WITH (UPDLOCK) WHERE ...
- TABLOCKX no need for explanations
The real question is what are you trying to achieve? Playing with lock hints w/o an absolute complete 110% understanding of the locking semantics is begging for trouble…
After OP edit:
I would like to select rows from a table and prevent the data in that
table from being modified while I am processing it.
The you should use one of the higher transaction isolation levels. REPEATABLE READ will prevent the data you read from being modified. SERIALIZABLE will prevent the data you read from being modified and new data from being inserted. Using transaction isolation levels is the right approach, as opposed to using query hints. Kendra Little has a nice poster exlaining the isolation levels.