Question about cache of sequence

Question

desc table temp1
id number(p.k)
comp number(5)
i have a trigger call BI_TEMP1
Trigger Type BEFORE EACH ROW
Triggering Event INSERT
begin
for c1 in (
select TEMP1_SEQ.nextval next_val
from dual
) loop
:new.ID := c1.next_val;
end loop;
end;
the sequence define as
Min Value 1
Max Value 999999999999999999999999999
Increment By 1
i have program which do insert to the table of 14 rows.
after the program finished i did :
select max(id) from temp1;
14
when i run the program again to insert those 14 rows again the sequence start from number 21
instead of 15
now i know that this is because of the cache tha equals 20.
my question is shall i use cache or not ?
i read about the cache option and i did not get the advantages of it
in which cases it is better to me to use cache and in which one is not?
thanks in advance

Answer 1

First of all, using a sequence is no guarentee that you'll end up without gaps! Transactions can be rolled back, etc, just like coffee can be spilt on your chequebook or whatever.
A cache for the sequence values is useful because it means that Oracle can store in memory the next sequence values, meaning that you cut down on the work needed to be done. If you don't have a cache, this is what happens:
1. Get the next value from the sequence.
2. use the value
3. Get the next value from the sequence.
4. use the value
5. Get the next value from the sequence.
6. use the value
etc...
However, if you have a cache of 5, this is what happens:
1. Get the next 5 values from the sequence and store in memory.
2. use the first value
3. use the second value
4. use the third value
5. use the fourth value
6. use the fifth value
7. Get the next 5 values from the sequence and store in memory.
8. use the first value
etc...
So a cache reduces the amount of calls to the sequence. However, as soon as the memory is wiped (a database bounce, shared_pool flush, etc) then the sequence numbers are gone, and the next time you ask for the next sequence value, it has to go back the sequence.