Estimate table size for last 4 years

Question

Hi,
I am on Oracle 10g
I need to estimate a table size for the last 4 years. So what I plan to do is get a count of data in the table for last 4 years and then multiply that value by avg_row_length to get the total size for 4 years. Is this technique correct or do I need to add some overhead?
Thanks

Answer 1

Yes, the technique is correct, but it is better to account for some overhead. I usually multiply the results by 10 :)
The most important thing to check is if there is any trend in data volumes. Was the count of records 4 years ago more or less equal to the last year? Is the business growing or steady? How fast is it growing? What are prospects for the future? Last year in not always 25% of last 4 years. It happens that last year is more than 3 other years added together.
The other, technical issue is internal organisation of data in Oracle datafiles. The famous PCTFREE. If you expact that the data will be updated then it is much better to keep some unused space in each database block in case some of your records get larger. This is much better for performance reasons. For example, you leave 10% of each database block free and when you update your record with longer value (like replace NULL column with actual 25-characters string) then your record still fits into the same block. You should account for this and add this to your estimates.
On the other hand, if your records get never updated and you load them in batch, then maybe they can be ORDERed before insert and you can setup a table with COMPRESS clause. Oracle COMPRESS clause has very little common with zip/gzip utilities, however it can bring you significant space savings.
Finally, there is no point to make estimates too accurate. They are just only estimates and the reality will be almost always different. In general, it is better to overestimate and have some disk space unused than underestimate and need to have people to deal with the issue. Disks are cheap, people on the project are expensive.