Estimate Database Size

Question

We are planning to use Oracle database (Oracle 10g in Unix). We are in the process of estimating the hard disk capacity(in GB) required based on the past data for each business process. But we are faced with following queries
1. Assuming the structure for a table is as follows :-
create table temp1(
Name varchar2(4),
Age Number(2),
Salary Number(8,2),
DOB Date)
The estimated records per year is assumed to be 500. How can we estimate the size that will be required for this table.
2. We are planning to allocate 20% space for indexes on each table. Is it ok?
3. Audit Logs (to keep track of changes made through update) :- Should it be kept in different partition/hard disk?
Is there anything else to consider. Is there a better way to estimate the hard disk capacity required in more accurate manner?
Our current database in Informix takes around 100GB per year, but there r lot of redundant data and due to business process change we cannot take that into consideration.
Kindly guide. Thanks in advance.

Answer 1

Well you can estimate the size of a table by estimating the average row size then multiplying this by the expected number of rows then add in overhead.
3 for the row header + 4 for name + 2 for age + 5 for Salary + 7 for a date + 1 for each column null/length indicator is about 25 bytes per row (single byte character set) * 500 rows * 1.20 (20% overhead) = small.
The overhead is the fixed block header, the ITL, the row table (2 bytes per row) and the pctfree. You can estimate this but I just said 20% which is probably a little high.
The total space needed by indexes often equals or surpassed the space needed by tables. You need to know the design and be pretty sure additional indexes will not be necessary for performance reasons before you allocate such a small percentage of the table space to index space.
Where you keep audit tables or extracts is totally dependent on your disk setup.
HTH -- Mark D Powell --