Bulk Fetch From an Oracle Sequence

Question

I am trying to get a range of sequence values from an Oracle sequence.
I am using the option as show below using query
SELECT SEQUENCE_NAME.NEXTVAL FROM SYS.DUAL CONNECT BY LEVEL <= 10.
The above SQL gets 10 sequence value.
I just wanted to to check, if the implementation below is safe in a Multi User Environment?
Is the statement show below atomic. i.e. Multi parallel execution of the same function; Would it cause any inconsistencies?
EXECUTE IMMEDIATE 'SELECT SEQUENCE_NAME.NEXTVAL ' ||
  'FROM SYS.DUAL CONNECT BY LEVEL <= ' || TO_CHAR(i_quantity)
  BULK COLLECT INTO v_seq_list;
FUNCTION select_sequence_nextval_range(
   i_quantity      IN  INTEGER)
RETURN INTEGER IS
  o_nextval INTEGER;
  v_seq_list sequence_list;
BEGIN
  EXECUTE IMMEDIATE 'SELECT SEQUENCE_NAME.NEXTVAL ' ||
  'FROM SYS.DUAL CONNECT BY LEVEL <= ' || TO_CHAR(i_quantity)
  BULK COLLECT INTO v_seq_list;
  -- Get the first poid value.
  o_nextval := v_seq_list(1);
  RETURN o_nextval;
END select_sequence_nextval_range

Answer 1

Acquire Lock
You acquire a lock on a sequence? That's news to me - please post the code that does that. I certainly hope you don' t mean you are directly accessing the SYS.SEQ$ table to lock the row for that sequence - it isn't nice to mess with Oracle's tables!
For couple of JAVA/C applications the usage of sequence number is pretty big. Could be 100,000 for one single application processing.
How does that correlate with your previous statement that you get 10 at a time?
Sequences aren't designed for use cases that require gap-free sets of numbers or for use cases that require consecutive sets of numbers.
We wanted to implement the range get of sequence using a different mechanism.
For few other applications; we just need one sequence number for the application processing. So we use the select seq.nextval to get the value. So the same sequence number needs to serve the role of giving a single value as well as a consecutive range of values.
Then you may need to consider using your own table to track the chunks that need to be allocated. You would use a scheme similar to what Greg.Spall discussed except you would keep the 'chunk' data in your own table.
I'm not talking about using your own table to control actual one-by-one sequence number generation - that is a very bad idea. But if you need to work with large ranges that are allocated infrequently there is nothing wrong with using your own function and your own table to keep track of those allocations.
The 'one by one' number generation would be handled by an actual sequence. The generation of a 'start value' and an 'end value' would be handled by accessing your custom table. Each row in that table would have 'start_value' and 'available_numbers' colulmns.
Your function would take a parameter for how many numbers you need. For just one number the function would call the sequence.nextval and return that along with a count of '1'.
For a range the function would:
1. find a row in the table with an 'available_numbers' value large enough to satisfy the request,
2. lock the row for update
3. capture the 'start_value' for return to the user
4. adjust both the 'start_value' and 'available_numbers' values to account for the range being allocated
5. update the table and commit
6. return the 'start_value' and 'number_allocated' to the user (number_allocated might be LESS than requested perhaps)
The above is a viable solution ONLY if the frequency of allocation and the size of allocation avoids the serialization issues associated with trying to allocate your own sequence numbers.
Those issues can be somewhat mitigated by having the table store multiple rows with each row having a large chunk of values that can be allocated. Then your function query can get the first 'unlocked' row and avoid serializing just because one row is currently locked.