I have an ASP.Net webpage where the user selects a row for editing. I want to use the row lock on that row and once the user finishes the editing and updates another user can edit that row i.e. How can I use rowlock so that only one user can edit a row?
Thank you
You can't lock a row like that using DB engine locks.
Most other strategies would rely on keeping the connection open (such as sp_getapplock) and this is nonsensical in web apps.
Even if you set a flag on the row, what happens if the user simply closes the browser mid-edit?
I'd suggest using a timestamp/rowversion column to detect changes to the row in other sessions.
Here i am giving two set of sql to row lock during select statement.
BEGIN TRAN
SELECT *
FROM authors AU
WITH (HOLDLOCK, ROWLOCK)
WHERE AU.au_id = '274-80-9391'
/* Do all your stuff here while the row is locked */
COMMIT TRAN
The HOLDLOCK hint politely asks SQL Server to hold the lock until you commit the transaction. The ROWLOCK hint politely asks SQL Server to lock only this row rather than issuing a page or table lock.
Be aware that if lots of rows are affected, either SQL Server will take the initiative and escalate to page locks, or you'll have a whole army of row locks filling your server's memory and bogging down processing.
another one
SELECT id From mytable WITH (ROWLOCK, UPDLOCK) WHERE id = 1
another good link i want to share with you on lock. https://www.mssqltips.com/sqlservertip/1257/processing-data-queues-in-sql-server-with-readpast-and-updlock/
thanks
Related
For testing, I am trying to simulate a condition in which a query from our web application to our SQL Server backend would timeout. The web application is configured so this happens if the query runs longer than 30 seconds. I felt the easiest way to do this would be to take and hold an exclusive lock on the the table that the web application wants to query. As I understand it, an exclusive lock should prevent any additional locks (even the shared locks taken by a SELECT statement).
I used the following methodology:
CREATE A LONG-HELD LOCK
Open a first query window in SSMS and run
BEGIN TRAN;
SELECT * FROM MyTable WITH (TABLOCKX);
WAITFOR DELAY '00:02:00';
ROLLBACK;
(see https://stackoverflow.com/a/25274225/2824445 )
CONFIRM THE LOCK
I can EXEC sp_lock and see results with ObjId matching MyTable, Type of TAB, Mode of X
TRY TO GET BLOCKED BY THE LOCK
Open a second query window in SSMS and run SELECT * FROM MyTable
I would expect this to sit and wait, not returning any results until after the lock is released by the first query. Instead, the second query returns with full results immediately.
STUFF I TRIED
In the second query window, if I SET TRANSACTION ISOLATION LEVEL SERIALIZABLE, then the second query waits until the first completes as expected. However, the point is to simulate a timeout in our web application, and I do not have any easy way to alter the transaction isolation level of the web application's connections away from the default of READ COMMITTED.
In the first window, I tried modifying the table's values inside the transaction. In this case, when the second query returns immediately, the values it shows are the unmodified values.
Figured it out. We had READ_COMMITTED_SNAPSHOT turned on, which is how the second query was able to return the previous, unmodified values in part 2 of "Stuff I tried". I was able to determine this with SELECT is_read_committed_snapshot_on FROM sys.databases WHERE name = 'MyDatabase'. Once it was turned off with ALTER DATABASE MyDatabase SET READ_COMMITTED_SNAPSHOT OFF, I began to see the expected behavior in which the second query would wait for the first to complete.
My application runs a nightly purge process to delete old records from the primary tables in my OLTP application. I was experiencing lock escalation during the purge process which was blocking concurrent inserts into the table, so I modified the purge procedure to loop through and delete records in blocks of 4900 which should be well below SQL Server's lock escalation threshold of 5000. While lock escalation was much reduced, SQL Server Profiler still reports occasional lock escalation on the following DELETE statement in the loop:
-- outer loop increments #BatchMinId and #BatchMaxId variables
BEGIN TRAN
-- limit is set at 4900
DELETE TOP (#limit) h
OUTPUT DELETED.ChildTable1Id,
DELETED.ChildTable2Id,
DELETED.ChildTable3Id,
DELETED.ChildTable4Id
INTO #ChildRecordsToDelete
FROM MainTable h WITH (ROWLOCK)
WHERE h.Id >= #BatchMinId AND h.Id <= #BatchMaxId AND h.Id < #MaxId AND
NOT EXISTS (SELECT 1 FROM OtherTable ot WHERE ot.Id = h.Id);
-- delete from ChildTables 1-4 (no additional references to MainTable)
COMMIT TRAN;
-- end loop
The "IntegerData2" column in SQL Server Profiler for the reported lock escalation events (which is supposed to be the escalated lock count) ranges from 10197 to 10222 which does not look close to any multiple of 4900 (my purge batch size) plus any multiple of 1250 (number of additional locks SQL Server may take before attempting escalation).
Given that I am explicitly limiting the DELETE statement to 4900 rows, how are more locks ever being taken, especially to the point that SQL Server is escalating to a table lock? I would like to understand this before I resort to disabling lock escalation altogether on this table.
I can't comment on your question since I don't have enough reputation on this web site, so I'm commenting here.
I had a similar issue with a cleanup task running at night. The delete statement was locked by the "GHOST CLEANUP".
Here have a look at this :
SQL Server Lock Timeout Exceeded Deleting Records in a Loop
Hope this help.
One weird solution that I found at the time was :
1) Insert the record to keep in another table with same structure. (Copy)
2) Truncate table to clean
3) Insert back data to keep from the copy into the now empty table.
4) Truncate copy table to release space.
This trick was faster to cleanup then the delete itself, because the deletion was done in a split second because of truncate. Somehow the cost of insertion was less expensive then deletion one.
But still, I would recommend to avoid this solution. You could also reduce the chunk between 100 to 500. This increase time the cleanup takes, but you are less likely to have the lock escalation.
I have a program that connects to an Oracle database and performs operations on it. I now want to adapt that program to also support an SQL Server database.
In the Oracle version, I use "SELECT FOR UPDATE WAIT" to lock specific rows I need. I use it in situations where the update is based on the result of the SELECT and other sessions can absolutely not modify it simultaneously, so they must manually lock it first. The system is highly subject to sessions trying to access the same data at the same time.
For example:
Two users try to fetch the row in the database with the highest priority, mark it as busy, performs operations on it, and mark it as available again for later use.
In Oracle, the logic would go basically like this:
BEGIN TRANSACTION;
SELECT ITEM_ID FROM TABLE_ITEM WHERE ITEM_PRIORITY > 10 AND ITEM_CATEGORY = 'CT1'
ITEM_STATUS = 'available' AND ROWNUM = 1 FOR UPDATE WAIT 5;
UPDATE [locked item_id] SET ITEM_STATUS = 'unavailable';
COMMIT TRANSACTION;
Note that the queries are built dynamically in my code. Also note that when the previously most favorable row is marked as unavailable, the second user will automatically go for the next one and so on. Furthermore, different users working on different categories will not have to wait for each other's locks to be released. Worst comes to worst, after 5 seconds, an error would be returned and the operation would be cancelled.
So finally, the question is: how do I achieve the same results in SQL Server? I have been looking at locking hints which, in theory, seem like they should work. However, the only locks that prevents other locks are "UPDLOCK" AND "XLOCK" which both only work at a table level.
Those locking hints that do work at a row level are all shared locks, which also do not satisfy my needs (both users could lock the same row at the same time, both mark it as unavailable and perform redundant operations on the corresponding item).
Some people seem to add a "time modified" column so sessions can verify that they are the ones who modified it, but this sounds like there would be a lot of redundant and unnecessary accesses.
You're probably looking forwith (updlock, holdlock). This will make a select grab an exclusive lock, which is required for updates, instead of a shared lock. The holdlock hint tells SQL Server to keep the lock until the transaction ends.
FROM TABLE_ITEM with (updlock, holdlock)
As documentation sayed:
XLOCK
Specifies that exclusive locks are to be taken and held until the
transaction completes. If specified with ROWLOCK, PAGLOCK, or TABLOCK,
the exclusive locks apply to the appropriate level of granularity.
So solution is using WITH(XLOCK, ROWLOCK):
BEGIN TRANSACTION;
SELECT ITEM_ID
FROM TABLE_ITEM
WITH(XLOCK, ROWLOCK)
WHERE ITEM_PRIORITY > 10 AND ITEM_CATEGORY = 'CT1' AND ITEM_STATUS = 'available' AND ROWNUM = 1;
UPDATE [locked item_id] SET ITEM_STATUS = 'unavailable';
COMMIT TRANSACTION;
In SQL Server there are locking hints but they do not span their statements like the Oracle example you provided. The way to do it in SQL Server is to set an isolation level on the transaction that contains the statements that you want to execute. See this MSDN page but the general structure would look something like:
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
BEGIN TRANSACTION;
select * from ...
update ...
COMMIT TRANSACTION;
SERIALIZABLE is the highest isolation level. See the link for other options. From MSDN:
SERIALIZABLE Specifies the following:
Statements cannot read data that has been modified but not yet
committed by other transactions.
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.
Have you tried WITH (ROWLOCK)?
BEGIN TRAN
UPDATE your_table WITH (ROWLOCK)
SET your_field = a_value
WHERE <a predicate>
COMMIT TRAN
SQL Server 2008 R2 (Data Center edition - I think)
I have a very specific requirement for the database.
I need to insert a row marked with timestamp [ChangeTimeStamp]. Timestamp value is passed as a parameter. Timestamp has to be unique.
Two processes can insert values at the same time, and I happen to run into duplicate key insertion once in a while. To avoid this, I am trying:
declare #maxChangeStamp bigint
set transaction isolation level read committed
begin transaction
select #maxChangeStamp = MAX(MaxChangeTimeStamp) from TSMChangeTimeStamp
if (#maxChangeStamp > #changeTimeStamp)
set #maxChangeStamp = #maxChangeStamp + 1
else
set #maxChangeStamp = #changeTimeStamp
update TSMChangeTimeStamp
set MaxChangeTimeStamp = #maxChangeStamp
commit
set #changeTimeStamp = #maxChangeStamp
insert statment
REPEATABLE READ - causes deadlock
READ COMMITTED - causes duplicate key inserts
#changeTimeStamp is my parameter.
TSMChangeTimeStamp holds only one value.
If anyone has a good idea how to solve this I will appreciate any help.
You don't read-increment-update, this will fail no matter what you try. Alway update and use the OUTPUT clause to the new value:
update TSMChangeTimeStamp
set MaxChangeTimeStamp += 1
output inserted.MaxChangeTimeStamp;
You can capture the output value if you need it in T-SQL. But although this will do what you're asking, you most definitely do not want to do this, specially on a system that is high end enough to run DC edition. Generating the next timestamp will place an X lock on the timestamp resource, and thus will prevent every other transaction from generating a new timestamp until the current transaction commits. You achieve complete serialization of work with only one transaction being active at a moment. The performance will tank to the bottom of the abyss.
You must revisit your requirement and come up with a more appropriate one. As it is now your requirement can also be expressed as 'My system is too fast, how can I make is really really really slow?'.
Inside the transaction, the SELECT statement will acquire a shared lock if the mode is not READ COMMITTED or snapshot isolation. If two processes both start the SELECT at the same time, they will both acquire a shared lock.
Later, the UPDATE statement attempts to acquire an exclusive lock (or update lock). Unfortunately, neither one can acquire an exclusive lock, because the other process has a shared lock.
Try using the WITH (UPDLOCK) table hint on the SELECT statement. From MSDN:
UPDLOCK
Specifies that update locks are to be taken and held until the
transaction completes. UPDLOCK takes update locks for read operations
only at the row-level or page-level. If UPDLOCK is combined with
TABLOCK, or a table-level lock is taken for some other reason, an
exclusive (X) lock will be taken instead.
When UPDLOCK is specified, the READCOMMITTED and READCOMMITTEDLOCK
isolation level hints are ignored. For example, if the isolation level
of the session is set to SERIALIZABLE and a query specifies (UPDLOCK,
READCOMMITTED), the READCOMMITTED hint is ignored and the transaction
is run using the SERIALIZABLE isolation level.
For example:
begin transaction
select #maxChangeStamp = MAX(MaxChangeTimeStamp) from TSMChangeTimeStamp with (updlock)
Note that update locks may be promoted to a table lock if there is no index for your table (Microsoft KB article 179362).
Explicitly requesting an XLOCK may also work.
Also note your UPDATE statement does not have a WHERE clause. This causes the UPDATE to lock and update every record in the table (if applicable in your case).
When I perform a select/Insert query, does SQL Server automatically create an implicit transaction and thus treat it as one atomic operation?
Take the following query that inserts a value into a table if it isn't already there:
INSERT INTO Table1 (FieldA)
SELECT 'newvalue'
WHERE NOT EXISTS (Select * FROM Table1 where FieldA='newvalue')
Is there any possibility of 'newvalue' being inserted into the table by another user between the evaluation of the WHERE clause and the execution of the INSERT clause if I it isn't explicitly wrapped in a transaction?
You are confusing between transaction and locking. Transaction reverts your data back to the original state if there is any error. If not, it will move the data to the new state. You will never ever have your data in an intermittent state when the operations are transacted. On the other hand, locking is the one that allows or prevents multiple users from accessing the data simultaneously. To answer your question, select...insert is atomic and as long as no granular locks are explicitly requested, no other user will be able to insert while select..insert is in progress.
John, the answer to this depends on your current isolation level. If you're set to READ UNCOMMITTED you could be looking for trouble, but with a higher isolation level, you should not get additional records in the table between the select and insert. With a READ COMMITTED (the default), REPEATABLE READ, or SERIALIZABLE isolation level, you should be covered.
Using SSMS 2016, it can be verified that the Select/Insert statement requests a lock (and so most likely operates atomically):
Open a new query/connection for the following transaction and set a break-point on ROLLBACK TRANSACTION before starting the debugger:
BEGIN TRANSACTION
INSERT INTO Table1 (FieldA) VALUES ('newvalue');
ROLLBACK TRANSACTION --[break-point]
While at the above break-point, execute the following from a separate query window to show any locks (may take a few seconds to register any output):
SELECT * FROM sys.dm_tran_locks
WHERE resource_database_id = DB_ID()
AND resource_associated_entity_id = OBJECT_ID(N'dbo.Table1');
There should be a single lock associated to the BEGIN TRANSACTION/INSERT above (since by default runs in an ISOLATION LEVEL of READ COMMITTED)
OBJECT ** ********** * IX LOCK GRANT 1
From another instance of SSMS, open up a new query and run the following (while still stopped at the above break-point):
INSERT INTO Table1 (FieldA)
SELECT 'newvalue'
WHERE NOT EXISTS (Select * FROM Table1 where FieldA='newvalue')
This should hang with the string "(Executing)..." being displayed in the tab title of the query window (since ##LOCK_TIMEOUT is -1 by default).
Re-run the query from Step 2.
Another lock corresponding to the Select/Insert should now show:
OBJECT ** ********** 0 IX LOCK GRANT 1
OBJECT ** ********** 0 IX LOCK GRANT 1
ref: How to check which locks are held on a table