Delete statement was very slow in Oracle

Question

I have a table with about 100k records and I want to delete some rows, The problem is that the DELETE statement is running very slowly - it didn't finish in 30 minutes. But the select statement was return in 1 second.

The SELECT statement is as follows:

select * from daily_au_by_service_summary 
    where summary_ts >= to_date('09-04-2012','dd-mm-yyyy') 
    order by summary_ts desc;

and DELETE statement is as follows:

delete from daily_au_by_service_summary 
    where summary_ts > to_date('09-04-2012','dd-mm-yyyy');

This table have the only index at summary_ts.

What could be the reason?

EDIT: the table was locked by many sessions:

SESSION_ID ORACLE_USERNAME                OS_USER_NAME                   OBJECT OWNER                   OBJECT_NAME                                                                                                                      OBJECT_TYPE         LOCKED_MODE
---------- ------------------------------ ------------------------------ ------------------------------ -------------------------------------------------------------------------------------------------------------------------------- ------------------- -----------
       213 T03RPT                         elou                           T03RPT                         DAILY_AU_BY_SERVICE_SUMMARY                                                                                                      TABLE                         3 
       203 T03RPT                         elou                           T03RPT                         DAILY_AU_BY_SERVICE_SUMMARY                                                                                                      TABLE                         3 
       202 T03RPT                         elou                           T03RPT                         DAILY_AU_BY_SERVICE_SUMMARY                                                                                                      TABLE                         3 
       190 T03RPT                         elou                           T03RPT                         DAILY_AU_BY_SERVICE_SUMMARY                                                                                                      TABLE                         3 
       189 T03RPT                         elou                           T03RPT                         DAILY_AU_BY_SERVICE_SUMMARY                                                                                                      TABLE                         3 
       188 T03RPT                         elou                           T03RPT                         DAILY_AU_BY_SERVICE_SUMMARY                                                                                                      TABLE                         3 
       187 T03RPT                         elou                           T03RPT                         DAILY_AU_BY_SERVICE_SUMMARY         

How to kill these sessions?

The problem had been resolved after I killed the sessions which locks the table, thanks all for the help. –

Answer 1

Obviously, a delete operation will take longer than a select, but that doesn't account for the difference you see.

It sounds like additional code is being run upon the delete, which indecates there may be triggers on the table that are also running. Can you check this?

Answer 2

To delete means to change the table's content. And this means, that after each deleted row all indexes must be updated and all foreign-key references must be checked. This can take a very long time!

Maybe this helps:

Make a copy of that table without any references, triggers and additional indexes. Then do this:

insert into new_table (field1, field2, ...) values (
    select field1, field2, ...
    from daily_au_by_service_summary 
    where summary_ts < to_date('09-04-2012','dd-mm-yyyy') 
);

If the fields in the tabels are defined in identical order, this might work too:

insert into new_table values (
    select *
    from daily_au_by_service_summary 
    where summary_ts < to_date('09-04-2012','dd-mm-yyyy') 
);

After that:

truncate daily_au_by_service_summary

and then:

insert into daily_au_by_service_summary (field1, field2, ...) values (
    select field1, field2, ...
    from new_table; 
);

New Table is not needed any longer:

drop new_table;

Answer 3

When DML operations take a long time, create new table with the remaining rows and drop the previous table, instead of deleting.

I mean,

create table NEW_TABLE as
select * from daily_au_by_service_summary  
where summary_ts <= to_date('09-04-2012','dd-mm-yyyy'); 

This will be faster, especially when you are deleting a substantial number of rows. (%10 of total rows, for example.)

Answer 4

There can be many reasons:

• Server load (unlikely because the SELECT is fast)
• Triggers (see here how to list them for a table).
• Foreign keys (List of foreign keys and the tables they reference)
• A lot of data in each row (LOBs, many columns).
• Someone is locking rows in the table that you'd like to delete (or the whole table). See this blog post how to list locks. This discussion might also help.

If the foreign keys are the problem, the usual solution is to add indexes on the foreign column: For each delete, Oracle needs to check whether this would violate a foreign key relation.