Inserting large number of records without locking the table

Question

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

Answer 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.

1. Break up large batch operations into several smaller operations (i.e. use a smaller batch size)
2. 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 

3. 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.