SELECT FOR UPDATE with SQL Server

Question

I\'m using a Microsoft SQL Server 2005 database with isolation level READ_COMMITTED and READ_COMMITTED_SNAPSHOT=ON.

Now I want to use:

Answer 1

The full answer could delve into the internals of the DBMS. It depends on how the query engine (which executes the query plan generated by the SQL optimizer) operates.

However, one possible explanation (applicable to at least some versions of some DBMS - not necessarily to MS SQL Server) is that there is no index on the ID column, so any process trying to work a query with 'WHERE id = ?' in it ends up doing a sequential scan of the table, and that sequential scan hits the lock which your process applied. You can also run into problems if the DBMS applies page-level locking by default; locking one row locks the entire page and all the rows on that page.

There are some ways you could debunk this as the source of trouble. Look at the query plan; study the indexes; try your SELECT with ID of 1000000 instead of 1 and see whether other processes are still blocked.