Delete large amount of data in sql server

Question

suppose that I have a table with 10000000 record. What is difference between this two solution?

1- delete data like :

DELETE FROM MyTable

2- delete all of data with a application row by row :

DELETE FROM MyTable WHERE ID = @SelectedID

Is the first solution has best performance? what is the impact on log and performance?

Answer 1

The first has clearly better performance.

When you specify DELETE [MyTable] it will simply erase everything without doing checks for ID. The second one will waste time and disk operation to locate a respective record each time before deleting it.

It also gets worse because every time a record disappears from the middle of the table, the engine may want to condense data on disk, thus wasting time and work again.

Maybe a better idea would be to delete data based on clustered index columns in descending order. Then the table will basically be truncated from the end at every delete operation.

Answer 2

Option 1 will create a very large transaction and have a big impact on the log / performance, as well as escalating locks so that the table will be unavailable. Option 2 will be slower, although it will generate less impact on the log (assuming bulk / full mode)

If you want to get rid of all the data, Truncate Table MyTable would be faster than both, although it has no facility to filter rows, it does a meta data change at the back and basically drops the IAM on the floor for the table in question.

Answer 3

The best performance for clearing a table would bring TRUNCATE TABLE MyTable. See http://msdn.microsoft.com/en-us/library/ms177570.aspx for more verbose explaination

Answer 4

If you have that manny records in your table and you want to delete them all, you should consider truncate table instead of delete from table. It will be much faster, but be aware that it cannot activate a trigger.

See for more details (this case sql server 2000): http://msdn.microsoft.com/en-us/library/aa260621%28SQL.80%29.aspx

Deleting the table within the application row by row will end up in long long time, as your dbms can not optimize anything, as it doesn't know in advance, that you are going to delete everything.

Answer 5

The first will delete all the data from the table and will have better performance that your second who will delete only data from a specific key.

Now if you have to delete all the data from the table and you don't rely on using rollback think of the use a truncate table

Answer 6

If you need to restrict to what rows you need to delete and not do a complete delete, or you can't use TRUNCATE TABLE (e.g. the table is referenced by a FK constraint, or included in an indexed view), then you can do the delete in chunks:

DECLARE @RowsDeleted INTEGER
SET @RowsDeleted = 1
SET ROWCOUNT 10000 -- delete 10,000 rows a time

WHILE (@RowsDeleted > 0)
    BEGIN
        DELETE FROM MyTable [WHERE .....] -- WHERE is optional
        SET @RowsDeleted = @@ROWCOUNT
    END

Generally, TRUNCATE is the best way and I'd use that if possible. But it cannot be used in all scenarios. Also, note that TRUNCATE will reset the IDENTITY value for the table if there is one.

Edit:
As per comment, assuming you are using SQL 2005 or later, you should start using the TOP condition instead of SET ROWCOUNT.

DECLARE @RowsDeleted INTEGER
SET @RowsDeleted = 1

WHILE (@RowsDeleted > 0)
    BEGIN
        DELETE TOP (10000) FROM MyTable [WHERE .....] -- WHERE is optional
        SET @RowsDeleted = @@ROWCOUNT
    END