"Create Table from select Query" Vs "Insert into"

Question

Hi
Schenaio:
My Select Query returns more than 10 million records, these records needs to be inserted into another table.
Approach 1:
I created table called TABLE1, and inserted the records using INSERT statement as a batch (batch size is 5000).
Approach 2:
I create table like,
CREATE TABLE TABLE1 AS <SELECT QUERY>
Here Apporach-1 took almost 40 minutes to complete the insert but Approach-2 took only 6 minutes.
If anybody knows why it is? And is there any way to improve the performance of Approach-1?.
Thanks
Nidhi

Answer 1

Most "batch" methods execute the same query multiple times. Row filtering is done after the rows are fetched from the source. The process of fetching all the rows could be a FullTableScan.
Therefore, a FullTableScan is executed for each batch of 5000 rows.
However, your query and batch definitions may well be different. We haven't seen the query and the execution plan.
Another point : How are you "filtering" the rows (i.e the second execution inserts rows 5001 to 10000 and does not attempt to reinsert rows 1 to 5000) ?
What is the overhead imposed by the filter ? (does the third execution have to exclude rows 1 to 10000 and inserts rows 10001 to 15000 and so on)
Hemant K Chitale