Slow delete whether or not deleted singly or as a group

Question

During performance analysis for my project I found some code where deletes from table y (I will call it), were being deleted in a loop, thus deleting one record at a time where there are approx. 87,000 records to delete.  Table y has 200 million records.  I rewrote the code to do a delete by internal table, i.e. DELETE y FROM TABLE t_y, to delete all 87,000 records at once.  The strange thing that is happening is that the deletes take about the same time whether it is a one by one delete or all at once by internal table, ~40ms per deleted record.  The delete by internal table shows in the job statistics and ST05 that it is processing 256 records at a time, so there are 340 items in the trace for the DELETE (340 * 256 ~= 87,000).  All of the reads, updates, and selects take about 1ms per record.  The internal table (t_y) is typed exactly like the transparent table.  All of the records requested to be deleted are in the table y and there are no duplicates in the internal table.
Old Code:
LOOP AT t_y.
     DELETE y FROM t_y.
ENDLOOP.
New Code:
DELETE y FROM TABLE t_y.
It takes approx. 1 hour to delete 87,000 records.  So the question is why deleting all records has the same speed as deleting one by one.  Secondarily, why is the system only sending 256 ercords at a time to be deleted.
Thanks,
Andy Goerdel

Answer 1

Hi Andy,
> one by one delete or all at once by internal table, ~40ms per deleted record.
> updates, and selects take about 1ms per record
first of all the delete has to touch the table and all indexes which is not the case for select and not necessarily for the update.
So depending on the number of indexes defined a delete has to be slower than a select or an update (which does not touch all rows) because it has to do more i/o.
> It takes approx. 1 hour to delete 87,000 records.  So the question is why deleting all records has the same speed as
> deleting one by one. 
that is an interesting one. One should assume that the array processing should be sligthly faster... .
> Secondarily, why is the system only sending 256 records at a time to be deleted.
The SAP application server communicates in blocks (size depends... could be 32, 40, 64 or 128) with the database.
The key fields for the delete are filled in such a block until it is full. You have no influence here.
How much indexes are defined on your table? The 40 ms is most likely driven by physical i/o for table and index blocks.
The amount of i/o that needs to be done is the same for one by one or for array processing. The array interface only
redcues the number of calls. Obviously these savings are relatively small compared to the i/o that needs to be done
in any case.
With ST04 you can check how much physiscal i/os are redquired for your deletes.
Kind regards,
Hermann