SELECT FOR UPDATE with SQL Server
I'm using a Microsoft SQL Server 2005 database with isolation level READ_COMMITTED and READ_COMMITTED_SNAPSHOT=ON .
Now I want to use:
SELECT * FROM <tablename> FOR UPDATE
...so that other database connections block when trying to access the same row "FOR UPDATE".
I tried:
SELECT * FROM <tablename> WITH (updlock) WHERE id=1
...but this blocks all other connections even for selecting an id other than "1".
Which is the correct hint to do a SELECT FOR UPDATE as known for Oracle, DB2, MySql?
EDIT 2009-10-03:
These are the statements to create the table and the index:
CREATE TABLE example ( Id BIGINT NOT NULL, TransactionId BIGINT,
Terminal BIGINT, Status SMALLINT );
ALTER TABLE example ADD CONSTRAINT index108 PRIMARY KEY ( Id )
CREATE INDEX I108_FkTerminal ON example ( Terminal )
CREATE INDEX I108_Key ON example ( TransactionId )
A lot of parallel processes do this SELECT :
SELECT * FROM example o WITH (updlock) WHERE o.TransactionId = ?
EDIT 2009-10-05:
For a better overview I've written down all tried solutions in the following table:
mechanism | SELECT on different row blocks | SELECT on same row blocks -----------------------+--------------------------------+-------------------------- ROWLOCK | no | no updlock, rowlock | yes | yes xlock,rowlock | yes | yes repeatableread | no | no DBCC TRACEON (1211,-1) | yes | yes rowlock,xlock,holdlock | yes | yes updlock,holdlock | yes | yes UPDLOCK,READPAST | no | no I'm looking for | no | yes
Recently I had a deadlock problem because Sql Server locks more then necessary (page). You can't really do anything against it. Now we are catching deadlock exceptions... and I wish I had Oracle instead.
Edit: We are using snapshot isolation meanwhile, which solves many, but not all of the problems. Unfortunately, to be able to use snapshot isolation it must be allowed by the database server, which may cause unnecessary problems at customers site. Now we are not only catching deadlock exceptions (which still can occur, of course) but also snapshot concurrency problems to repeat transactions from background processes (which cannot be repeated by the user). But this still performs much better than before.
I have a similar problem, I want to lock only 1 row. As far as I know, with UPDLOCK option, SQLSERVER locks all the rows that it needs to read in order to get the row. So, if you don't define a index to direct access to the row, all the preceded rows will be locked. In your example:
Asume that you have a table named TBL with an id field. You want to lock the row with id=10 . You need to define a index for the field id (or any other fields that are involved in you select):
CREATE INDEX TBLINDEX ON TBL ( id )
And then, your query to lock ONLY the rows that you read is:
SELECT * FROM TBL WITH (UPDLOCK, INDEX(TBLINDEX)) WHERE id=10.
If you don't use the INDEX(TBLINDEX) option, SQLSERVER needs to read all rows from the beginning of the table to find your row with id=10 , so those rows will be locked.
You cannot have snapshot isolation and blocking reads at the same time. The purpose of snapshot isolation is to prevent blocking reads.
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