Hi everyone,

I come across an Oracle Reference Book with following statement :

Simple Index contains only one column of the corresponding row in the table.
Composite Indexes store more than one column of data for you to search, plus the ROWID of the corresponding row in the table. You can up to 32 columns in a composite index, but you may be restricted from including that many if the total size of all the columns you want in the index exceeds DB_BLOCK_SIZE / 3.

How to calculate the total size of all the columns ?
Why the db_block_size must divide by 3 ?

Thanks