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

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         

Answer 1

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.