HCC compression and UPDATES

Question

We use HCC QH compression for older partitions and we have a requirement to update some of these older partitions. From what I read in documents, those updated records will get converted to OLTP compression format. If this is the case, how do I convert these records back to HCC QH compression.
Are there any commands to find those records changed from HCC-QH to OLTP and commands to convert them back to HCC. I am thinking of implementing a weekly mainteance process to do those steps to locate OLTP and convert to HCC-QH.
- Sreeni

Answer 1

Hi Sreeni,
I would advise that you refer to the following link by Kerry Osborne to find you what level of compression a particular row has been compressed with.
http://kerryosborne.oracle-guy.com/2011/01/ehcc-and-the-get_compression_type-function/
As far as your problem goes the question is that how many rows in your partition are getting updated. If the number of rows are very few then it does not matter since only those rows will change their compression type from EHCC to OLTP. In case the number of rows are huge then you may want to consider the option of compressing the partition again using EHCC. As of now I am not aware of any option that would just compress a few rows in the partition and I think you would have to compress the entire partition again using the ALTER MOVE statement.
To resolve your problem I would suggest the following options.
Option 1: In case you have footprint columns in your table (updated time, insert time) etc you can use the update time column to check which of the rows have been updated and then again compress those partitions in your weekly maintenance job.
Option 2: Setup a trigger on your table on update and for every update you can just insert a row in another table. You can further refine this trigger by just adding one row for each partition since you already know your partition boundaries (example if there is already a row for 11/23/2012 you don't want to insert a row for 11/24/2012 since you already know that you have to compress this partition).
Triggers are expensive so I would advise that if the updates are not hurting you much you can very well not do anything about it. You first need to calculate the size of your partition before the update (after compression is done) and then after the update has been performed. This way you will be sure to know whether you need to weekly job of not.