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Delete statement was very slow in Oracle

Tags:

sql

oracle

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         
like image 429
Elvis Lou Avatar asked Apr 11 '12 07:04

Elvis Lou


People also ask

Why delete is taking long time in Oracle?

Deleting lots of rows can be slow. And there's a chance it'll take even longer because another session has locked the data you want to remove.

How can I make delete statement faster in SQL Server?

If you are deleting 95% of a table and keeping 5%, it can actually be quicker to move the rows you want to keep into a new table, drop the old table, and rename the new one. Or copy the keeper rows out, truncate the table, and then copy them back in.


3 Answers

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;
like image 191
Hubert Schölnast Avatar answered Oct 23 '22 04:10

Hubert Schölnast


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.

like image 28
Aaron Digulla Avatar answered Oct 23 '22 03:10

Aaron Digulla


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?

like image 1
James Wiseman Avatar answered Oct 23 '22 04:10

James Wiseman