There is a process that runs ever hour that has to insert into table1 but when the query below is running, the inserts are blocked. Can anyone explain why? I realize that the lock hints are just that, hints and SQL may chose to ignore them. table1 has 300m rows and I need to update the values in ColumnA. I’m doing it in chunks to help with the rollback in case the process has to be stopped, hence the while loop.
SET TRANSACTION ISOLATION LEVEL READ UNCOMMITTED
go
declare @start bigint
declare @end bigint
declare @max bigint
declare @step int
set @step = 50000
set @start = 17953095
set @end = @start + @step
set @max = @start + 2000000
while ( @end < @max )
begin
waitfor delay '00:00:10'
begin transaction
update [table1] with (ROWLOCK)
set [table1].[ColumnA] = [table2].[ColumnA]
from [table2] (nolock)
where [table2].[ColumnB] = [table1].[ColumnB]
and [table1].ID >= @start
and [table1].ID < @end
commit transaction
print @end
if @end >= @max
begin
break
end
set @start = @end
set @end = @end + @step
end
print @end
Can I do this another way so it doesn’t block the inserts or other updates? Nothing else will need to work with ColumnA other than my query above.
You can try using a smaller batch size. SQL Server escalates row locks straight to table locks when a certain threshold is exceeded, per MSDN:
Your current batch size of 50,000 is well in excess of that threshold.
The other thing to think about is whether you have suitable indices in place to suit your WHERE clause – make sure that is as optimal as possible.