SQL Delete Request spend significant time in "Index Delete" even if it affects 0 rows

Question

I use MSSQL Server 2008 and I have SQL request with simple condition which periodically deletes old records from table (~3 mil records in the table).

This request executed significant time (~ 10 second) even if it affects 0 rows.

This table has some indexes and in Actual Execution Plan I see that "Index Delete" operations consume all execution time.

Why SQL Server does a lot work on indexes if there no any rows affected by delete operation?

Update:

Request:

delete t
from Entity t
where t.Revision <= x
AND exists (
    select 1
    from Entity tt
    where tt.Id=t.Id
    and tt.Revision > t.Revision
) 

Actual execution plan XML: pastebin.com/up2E3iP1

Answer 1

The work is all doing the hash join. All the other costs are bogus.

The actual number of rows coming out of that is 0 but it estimates more.

The costs shown in the rest of the plan are based on the (incorrect) estimates.

You might find this performs better.

WITH T AS
(
SELECT *,
       ROW_NUMBER() OVER (PARTITION BY Id 
                              ORDER BY Revision DESC) AS RN
FROM Entity
)
DELETE FROM T
WHERE RN > 1
AND Revision <= 12586705

Answer 2

I've found joins to be much more performant that subqueries.

Try this

delete t
from Entity t
inner join Entity tt ON tt.Id=t.Id
where t.Revision <= x
and tt.Revision > t.Revision

Also, ensure you have an index on Id and Revision.

Answer 3

Don't forget that even though you aren't going to delete any records SQL still has to check to confirm that there are no records to delete. Your join between Entity t and Entity tt has a < in it which is going to require some extra work. Try running SET STATISTICS IO ON in your query window before running the delete. I'll bet there is a fair amount of IO going on even with the index. I assume you have an index on Id, Revision? If not you might try adding one.