Truncate performance

Question

Dear SQL Experts,
We are trying to delete 35 Million rows from a table with 60 Million rows. Table is non-partitioned, non-clustered, and no LOBs.
Here's what I'm doing:
1. creating a new table, using insert append to copy all the rows I want to retain from original table
2. truncate the original table
3. Import rows from new table to originial table.
My question is - Original table has indexes on it. Does dropping indexes help the truncate run faster? I know for delete it does... not sure if indices matter for truncate..Kindly, share if you think any other method is
more germane.
Thanks

Answer 1

Hi,
if the table is partitioned then it might be faster to insert rows using ALTER TABLE EXCHANGE PARTITION.
It would work even if there's only one partition (so one could partition the table just to make such operations faster,
and it won't affect anything else):
create table t1 (id number, y number, z number)
partition by range(id)(
  partition p values less than(maxvalue)
insert into t1
(id, y, z)
select level id, dbms_random.value y, dbms_random.value z
from dual
connect by level <= 1e5;
create index i$t1 on t1(id);
create table t2
as
select *
from t1
where y<=0.5;
truncate table t1;
alter table t1 exchange partition (p) with table t2;
I wonder if it's possible to do the same thing with a non-partitioned table.
Best regards,
Nikolay