Re: Better Delete method

There are several methods to do this:

1. Create a partitioned table, exchange partition with the table and then split partition on the required date. That would require an application downtime and partitioning license.
2. Use simple DELETE with ROWNUM combination and then commit each batch separately. It will take some time to do that.
3. Basically, the first as 2, but using BULK COLLECT INTO and FORALL instead of ROWNUM. That is usually quite a bit faster than 2, but a bit more complex to program.

Of course, there are several Wally-based methods available but I would not really try those unless your boss is a PHB like character.

Hello , this database version is 11.2.0.4 of Oracle Exadata.  A table(say TAB1) is there holding ~900 million rows with size ~222GB and it's not partitioned. It has two indexes , one with a three column composite index with size ~98Gb and other is the primary key on one column with size ~23GB. As a part of the requirement we need to delete/purge 50% of its data from this table. No referential constraints exist here. So I wanted to understand, out of the two below, which is the best method to opt for? or any other possible better option?

I can understand method-1 is achievable ONLINE, but is slower while method-2 will be faster. So if we can afford ~1-2hrs of downtime, is it good to go for method -2 as the delete approach. As because deleting 50% rows even in method-1 may also need a table move+index rebuild(which will again need downtime on 11.2) to lower the high water mark and make the indexes compact and back to normal. Please advise.

Method-1:-

steps- 1:

  In a cursor pick the ~450million rowids of the rows to be deleted  based on filter criteria;

step2:-

   Delete based on ROW_IDS in a bulk collect fashion with LIMIT 50K rows ids at oneshot and commit within loop.

Method-2:- 

  Step- 1

      Create a new table using CTAS a new table TAB1_BKP AS select * from TAB1 where (required filter criteria which will pick ~450 required rows);

  Step-2:-

     Create the composite index in parallel.to make it as fast as possible 

     Create the unique index on the same column as there in PK.

     Create the primary constraints  with NOVALIDATE(because creating it with validate may take a lot of time to validate existing data) using the above unique index (This operation should happen in seconds as the index is already created in the above step.)

   Rename the TAB1_BKP as TAB1 and TAB1 as TAB1_BKP(which can be served as backup for a few days and later dropped).

       

-- 
Mladen Gogala
Database Consultant
Tel: (347) 321-1217
https://dbwhisperer.wordpress.com