Why should we create index on the table after inserting data ?

Question

Please tell me the Reason, why should we create index on the table after inserting data .
while we can also create index on the table before insertion of the data.

Answer 1

The choice depends on a number of factors, the main being how many rows are going to be inserted in the table as a percentage of the existing rows, or the percentage growth.
Creating index after a table has been populated works better when the tables are large or the inserts are large for the following reasons
1. The sort and creation of index is more efficient when done in batch and written in bulk. So works faster.
2. When the index is being written blocks get acquired as more data gets written. So, when a large number of rows get inserted in a table that already has an index , the index data blocks start splitting / chaining. This increases the "depth" of the inverted b-tree makes and that makes the index less efficient on I/O. Creating index after data has been inserted allows Orale to create optical block distribution/ reduce splitting / chaining
3. If an index exists then it too is routed through the undo / redo processes. Thats an overhead which is avoided when you create index after populating the table.
Regards