问题
I am trying to insert 1,500,000 records into a table. Am facing table lock issues during the insertion. So I came up with the below batch insert.
DECLARE @BatchSize INT = 50000
WHILE 1 = 1
BEGIN
INSERT INTO [dbo].[Destination]
(proj_details_sid,
period_sid,
sales,
units)
SELECT TOP(@BatchSize) s.proj_details_sid,
s.period_sid,
s.sales,
s.units
FROM [dbo].[SOURCE] s
WHERE NOT EXISTS (SELECT 1
FROM dbo.Destination d
WHERE d.proj_details_sid = s.proj_details_sid
AND d.period_sid = s.period_sid)
IF @@ROWCOUNT < @BatchSize
BREAK
END
I have a clustered Index on Destination table (proj_details_sid ,period_sid ). NOT EXISTS part is just to restrict inserted records from again inserting into the table
Am I doing it right, will this avoid table lock ? or is there any better way.
Note : Time taken is more or less same with batch and without batch insert
回答1:
Lock escalation is not likely to be related to the SELECT part of your statement at all.
It is a natural consequence of inserting a large number of rows
Lock escalation is triggered when lock escalation is not disabled on the table by using the ALTER TABLE SET LOCK_ESCALATION option, and when either of the following conditions exists:
- A single Transact-SQL statement acquires at least 5,000 locks on a single nonpartitioned table or index.
- A single Transact-SQL statement acquires at least 5,000 locks on a single partition of a partitioned table and the ALTER TABLE SET LOCK_ESCALATION option is set to AUTO.
- The number of locks in an instance of the Database Engine exceeds memory or configuration thresholds.
If locks cannot be escalated because of lock conflicts, the Database Engine periodically triggers lock escalation at every 1,250 new locks acquired.
You can easily see this for yourself by tracing the lock escalation event in Profiler or simply trying the below with different batch sizes. For me TOP (6228) shows 6250 locks held but TOP (6229) it suddenly plummets to 1 as lock escalation kicks in. The exact numbers may vary (dependant on database settings and resources currently available). Use trial and error to find the threshold where lock escalation appears for you.
CREATE TABLE [dbo].[Destination]
(
proj_details_sid INT,
period_sid INT,
sales INT,
units INT
)
BEGIN TRAN --So locks are held for us to count in the next statement
INSERT INTO [dbo].[Destination]
SELECT TOP (6229) 1,
1,
1,
1
FROM master..spt_values v1,
master..spt_values v2
SELECT COUNT(*)
FROM sys.dm_tran_locks
WHERE request_session_id = @@SPID;
COMMIT
DROP TABLE [dbo].[Destination]
You are inserting 50,000 rows so almost certainly lock escalation will be attempted.
The article How to resolve blocking problems that are caused by lock escalation in SQL Server is quite old but a lot of the suggestions are still valid.
- Break up large batch operations into several smaller operations (i.e. use a smaller batch size)
- Lock escalation cannot occur if a different SPID is currently holding an incompatible table lock - The example they give is a different session executing
BEGIN TRAN
SELECT * FROM mytable (UPDLOCK, HOLDLOCK) WHERE 1=0
WAITFOR DELAY '1:00:00'
COMMIT TRAN
- Disable lock escalation by enabling trace flag 1211 - However this is a global setting and can cause severe issues. There is a newer option 1224 that is less problematic but this is still global.
Another option would be to ALTER TABLE blah SET (LOCK_ESCALATION = DISABLE) but this is still not very targeted as it affects all queries against the table not just your single scenario here.
So I would opt for option 1 or possibly option 2 and discount the others.
回答2:
Instead of checking the data exists in Destination, it seems better to store all data in temp table first, and batch insert into Destination
Reference: Using ROWLOCK in an INSERT statement (SQL Server)
DECLARE @batch int = 100
DECLARE @curRecord int = 1
DECLARE @maxRecord int
-- remove (nolock) if you don't want to have dirty read
SELECT row_number over (order by s.proj_details_sid, s.period_sid) as rownum,
s.proj_details_sid,
s.period_sid,
s.sales,
s.units
INTO #Temp
FROM [dbo].[SOURCE] s WITH (NOLOCK)
WHERE NOT EXISTS (SELECT 1
FROM dbo.Destination d WITH (NOLOCK)
WHERE d.proj_details_sid = s.proj_details_sid
AND d.period_sid = s.period_sid)
-- change this maxRecord if you want to limit the records to insert
SELECT @maxRecord = count(1) from #Temp
WHILE @maxRecord >= @curRecord
BEGIN
INSERT INTO [dbo].[Destination]
(proj_details_sid,
period_sid,
sales,
units)
SELECT proj_details_sid, period_sid, sales, units
FROM #Temp
WHERE rownum >= @curRecord and rownum < @curRecord + @batch
SET @curRecord = @curRecord + @batch
END
DROP TABLE #Temp
回答3:
I added (NOLOCK) your destination table -> dbo.Destination(NOLOCK). Now, You won't lock your table.
WHILE 1 = 1
BEGIN
INSERT INTO [dbo].[Destination]
(proj_details_sid,
period_sid,
sales,
units)
SELECT TOP(@BatchSize) s.proj_details_sid,
s.period_sid,
s.sales,
s.units
FROM [dbo].[SOURCE] s
WHERE NOT EXISTS (SELECT 1
FROM dbo.Destination(NOLOCK) d
WHERE d.proj_details_sid = s.proj_details_sid
AND d.period_sid = s.period_sid)
IF @@ROWCOUNT < @BatchSize
BREAK
END
回答4:
To do this you can use WITH (NOLOCK) in your select statement. BUT NOLOCK is not recommended on OLTP Databases.
来源：https://stackoverflow.com/questions/39404875/inserting-large-number-of-records-without-locking-the-table