improve oracle delete query performance

Question

This is my query, I plan to run it in batches of perhaps 5000 hence the rownum < 5000

delete my_table
where  rownum < 5000 
and    type = 'Happy'
and    id not in 
       ( select max_id
         from   ( select max(log_id) max_id
                  ,      object_id
                  ,      type
                  from   my_table
                  where  type = 'Happy'
                  group
                  by     id
                  ,      type
                )
        )

I want to delete happy records but keeping the maximum log id, per object id

I hope that makes sense.

Should I be using some sort of join to improve performance?

Answer 1

I think this might run faster as a correlated subquery:

Delete
    from my_table
    where type = 'Happy' and
          exists (select 1
                  from my_table t2
                  where t2.object_id = my_table.object_id and
                        t2.type = my_table.type and
                        t2.id > my_table.id
                 );

Then, an index on my_table(object_id, type, id) might also help this query.

Answer 2

Since you only care to delete ANY 5000 log entries for type = 'Happy', as long as its not the most recent for any object_id, then you can do something like this:

delete
from my_table
where log_id in (
    select log_id from (
        select log_id, 
            row_number() over (partition by object_id order by log_id desc) rnk
        from my_table
        where typ = 'Happy' 
        and rownum <= 5000
    )
    where rnk > 1
)

This is different from what you have because in your approach, you still need to calculate the max(id) per object across the entire table, which isn't necessary (and log tables can get very large). You just need to make sure you're not deleting the "newest" row (per object) of the 5000 batch rows. Personally, I prefer to setup log tables using partitions, but not everyone has this option.

Hope that helps.

Answer 3

You could simplify the query to:

delete my_table
where  rownum < 5000 
and    type = 'Happy'
and    id not in (select   max(log_id) max_id
                  from     my_table
                  where    type = 'Happy'
                  group by object_id, type)