SQL Batched Delete

Question

I have a table in SQL Server 2005 which has approx 4 billion rows in it. I need to delete approximately 2 billion of these rows. If I try and do it in a single transaction, the transaction log fills up and it fails. I don't have any extra space to make the transaction log bigger. I assume the best way forward is to batch up the delete statements (in batches of ~ 10,000?).

I can probably do this using a cursor, but is the a standard/easy/clever way of doing this?

P.S. This table does not have an identity column as a PK. The PK is made up of an integer foreign key and a date.

Solution 1

You can 'nibble' the delete's which also means that you don't cause a massive load on the database. If your t-log backups run every 10 mins, then you should be ok to run this once or twice over the same interval. You can schedule it as a SQL Agent job

try something like this:

DECLARE @count int
SET @count = 10000

    DELETE  FROM table1 
    WHERE table1id IN (
        SELECT TOP (@count) tableid
        FROM table1
        WHERE x='y'
    )

Solution 2

What distinguishes the rows you want to delete from those you want to keep? Will this work for you:

while exists (select 1 from your_table where <your_condition>)
delete top(10000) from your_table
where <your_condition>

Solution 3

In addition to putting this in a batch with a statement to truncate the log, you also might want to try these tricks:

• Add criteria that matches the first column in your clustered index in addition to your other criteria
• Drop any indexes from the table and then put them back after the delete is done if that's possible and won't interfere with anything else going on in the DB, but KEEP the clustered index

For the first point above, for example, if your PK is clustered then find a range which approximately matches the number of rows that you want to delete each batch and use that:

DECLARE @max_id INT, @start_id INT, @end_id INT, @interval INT
SELECT @start_id = MIN(id), @max_id = MAX(id) FROM My_Table
SET @interval = 100000  -- You need to determine the right number here
SET @end_id = @start_id + @interval

WHILE (@start_id <= @max_id)
BEGIN
     DELETE FROM My_Table WHERE id BETWEEN @start_id AND @end_id AND <your criteria>

     SET @start_id = @end_id + 1
     SET @end_id = @end_id + @interval
END

Solution 4

Sounds like this is one-off operation (I hope for you) and you don't need to go back to a state that's halfway this batched delete - if that's the case why don't you just switch to SIMPLE transaction mode before running and then back to FULL when you're done?

This way the transaction log won't grow as much. This might not be ideal in most situations but I don't see anything wrong here (assuming as above you don't need to go back to a state that's in between your deletes).

you can do this in your script with smt like:

ALTER DATABASE myDB SET RECOVERY FULL/SIMPLE

Alternatively you can setup a job to shrink the transaction log every given interval of time - while your delete is running. This is kinda bad but I reckon it'd do the trick.

Solution 5

Well, if you were using SQL Server Partitioning, say based on the date column, you would have possibly switched out the partitions that are no longer required. A consideration for a future implementation perhaps.

I think the best option may be as you say, to delete the data in smaller batches, rather than in one hit, so as to avoid any potential blocking issues.

You could also consider the following method:

1. Copy the data to keep into a temporary table
2. Truncate the original table to purge all data
3. Move everything from the temporary table back into the original table

Your indexes would also be rebuilt as the data was added back to the original table.

View more solutions

Comments

[Tom Ferguson]

Thanks for the answer, we have looked in to partitioning, but its not practical for us to implement it at the momentn (paritially due to this issue: support.microsoft.com/kb/924601). Regarding copying data to a temporary table: would ths operation require any less transaction log space than deleting rows?

[Tom Ferguson]

Yes, it is a one off operation :) Unfortunately, We're already using simple recovery, but even with simple recovery, the tlog (100GB) fills up when doing the delete in a single transaction.

[Jimmy Stenke]

What's worth mentioning here though is that it will invalidate any transactional backups by switching over to simple recovery. If that is not being used, then it's fine (and I actually use that way a lot) but otherwise a full or differential backup is needed afterwards to be able to use the transactional backups again.

[John Sansom]

Possibly yes because you would not need to issue a DELETE operation. Once you have created a copy of the table, you would TRUNCATE the source table and then copy only the data you wish to keep back to the source table. I would still recommend that you go with the batch deletion though, as you really want to have all operations logged in order to guarantee the consistency/recoverability of your database.

[Tom Ferguson]

The where condition would basically be: WHERE DateTimeInserted < DATEDIFF(d, GETDATE(), 5). I can give this a try, but I'm concerned that because this will loop ~200,000 times, the select statement will execute 200,000 times and on a 2-4 billion row table it could take up to half an hour (from experience). If this is the case, the statement will take 11 years to run! :)

[Stanislav Kniazev]

You can skip the select if you think it's expensive (just replace it with some simpler exit condition). As for transaction log growth, I think you can do some tricks with checkpoints within the loop with "truncate on checkpoint" option turned on.

[JohnIdol]

what about the 'alternative' solution/hack? :)

[Tom Ferguson]

I could be wrong, but I believe that if you have simple recovery mode, the space in the transaction log will be recovered after each transaction is completed. Therefore I don't think I need to do any truncating/shrinking etc.

[Akshay]

You may want to add some description beyond the code itself.