Methods of speeding up a huge DELETE FROM <table> with no clauses

Question

Using SQL Server 2005.

I am performing a huge DELETE FROM with no where clauses. It's basically equivalent to a TRUNCATE TABLE statement - except I'm not allowed to use TRUNCATE. The problem is the table is huge - 10 million rows, and it takes over an hour to complete. Is there any way of making it faster without:

• Using Truncate
• Disabling or dropping indexes?

The t-log is already on a separate disk.

Any suggestions welcome!

Answer 1

What you can do is batch deletes like this:

SELECT 'Starting' --sets @@ROWCOUNT
WHILE @@ROWCOUNT <> 0
    DELETE TOP (xxx) MyTable

Where xxx is, say, 50000

A modification of this, if you want to remove a very high percentage of rows...

SELECT col1, col2, ... INTO #Holdingtable
           FROM MyTable WHERE ..some condition..

SELECT 'Starting' --sets @@ROWCOUNT
WHILE @@ROWCOUNT <> 0
    DELETE TOP (xxx) MyTable WHERE ...

INSERT MyTable (col1, col2, ...)
           SELECT col1, col2, ... FROM #Holdingtable

Answer 2

You could use the TOP clause to get this done easily:

WHILE (1=1)
BEGIN
    DELETE TOP(1000) FROM table
    IF @@ROWCOUNT < 1 BREAK
END

Answer 3

I agree with the suggestions to batch your deletes into managable chunks if you aren't able to use TRUNCATE, and I like the drop/create suggestion for it's originality, but I'm curious about the following comment in your question:

It's basically equivalent to a TRUNCATE TABLE statement - except I'm not allowed to use TRUNCATE

I'm guessing the reason for this restriction has to do with the security that needs to be granted to directly truncate a table and the fact that it would allow you to truncate tables other than the one you are concerned with.

Assuming that is the case, I'm wondering if having a stored procedure created that uses TRUNCATE TABLE and uses "EXECUTE AS" would be considered a viable alternative to giving security rights necessary to truncate the table directly.

Hopefully, this would give you the speed you need while also addressing the security concerns that your company may have with adding your account to the db_ddladmin role.

Another advantage of using a stored procedure this way is that the stored procedure itself could be locked down so that only specific account(s) are allowed to use it.

If for some reason this is not an acceptable solution and your need to have the data in this table removed is something that needs to be done once a day/hour/etc, I would request that a SQL Agent job was created to truncate the table at a scheduled time each day.

Hope this helps!

Answer 4

Except truncate.. only delete in batches can help you.

You could drop the table and recreate it, with all constraints and indexes, off course. In Management Studio you have the option to script a table to drop and create, so it should be a trivial option. But this only if you're allowed to do DDL actions, which I see it's not really an option.

Answer 5

Since this question is such an important reference I'm posting this code that really helped me understand deleting with loops and also messaging within a loop to track progress.

The query is modified from this duplicate question. Credit to @RLF for the query base.

CREATE TABLE #DelTest (ID INT IDENTITY, name NVARCHAR(128)); -- Build the test table
INSERT INTO #DelTest (name) SELECT name FROM sys.objects;  -- fill from system DB
SELECT COUNT(*) TableNamesContainingSys FROM #deltest WHERE name LIKE '%sys%'; -- check rowcount
go
DECLARE @HowMany INT;
DECLARE @RowsTouched INT;
DECLARE @TotalRowCount INT;
DECLARE @msg VARCHAR(100);
DECLARE @starttime DATETIME 
DECLARE @currenttime DATETIME 

SET @RowsTouched = 1; -- Needs to be >0 for loop to start
SET @TotalRowCount=0  -- Total rows deleted so far is 0
SET @HowMany = 5;     -- Variable to choose how many rows to delete per loop
SET @starttime=GETDATE()

WHILE @RowsTouched > 0
BEGIN
   DELETE TOP (@HowMany)
   FROM #DelTest 
   WHERE name LIKE '%sys%';

   SET @RowsTouched = @@ROWCOUNT; -- Rows deleted this loop
   SET @TotalRowCount = @TotalRowCount+@RowsTouched; -- Increment Total rows deleted count
   SET @currenttime = GETDATE();
   SELECT @msg='Deleted ' + CONVERT(VARCHAR(9),@TotalRowCount) + ' Records. Runtime so far is '+CONVERT(VARCHAR(30),DATEDIFF(MILLISECOND,@starttime,@currenttime))+' milliseconds.'
   RAISERROR(@msg, 0, 1) WITH NOWAIT;  -- Print message after every loop. Can't use the PRINT function as SQL buffers output in loops.  

END; 
SELECT COUNT(*) TableNamesContainingSys FROM #DelTest WHERE name LIKE '%sys%'; -- Check row count after loop finish
DROP TABLE #DelTest;