How to remove gap in sequence?

Question

Hi,
SELECT MENU_ITEMID.NEXTVAL INTO :MENU_ITEM.M_ITEM_ID
FROM SYS.DUAL;
EXCEPTION
WHEN OTHERS THEN MESSAGE ('FAILED');
RAISE FORM_TRIGGER_FAILURE;
I have applied the above PRE-INSERT trigger on my MENU_ITEM FORM.
I was inserting values but when I was inserting the 53th item my PC was struck by a power failure, I did not commit or Rollback and next item when I rebooted my system and tried to insert the 53th item in my form it gave me a gap and jumped to 54th item directly.
Now How can I remove this gap?
I tried to insert the ids manually using INSERT command,
INSERT INTO MENU_ITEM VALUES (53, 'AAAA');
INSERT INTO MENU_ITEM VALUES (54, 'BBAAA');
INSERT INTO MENU_ITEM VALUES (55, 'UUAA');
but when I opened the application form then it showed the wrong order of id column.
like
53
55
54
and so on.
I deleted the newly inserted records using this command, DELETE from MENU_ITEM where M_ITEM_ID in(53,54,55);
COMMIT;
and then i queried the nextval of sequence.
and it was 55.
now I want tthe solution to solve this.
Please Help me that where I can find the solution?
Thanks In advance
Natalia

Answer 1

I'm afraid you do not understand how a SEQUENCE works.
A sequence is a "one-time" sheet. Once we have got a number through NEXTVAL the sequence can never give us that number again (unless we created the sequence with the CYCLE clause, in which case we'll see it again, eventually). NEXTVAL is a piece of DDL, so we cannot roll it back. We can only fill in the gaps manually, as you did. You, however, overshot and inserted a row with an ID 55 that was in advance of the last number assigned by the sequence.
Also note that the SEQUENCE number has a CACHE; this is the default behaviour, because it is memory efficient. As a side effect, if the database goes down, assigned numbers in the cache may be lost. We can avoid this with the NOCACHE option, but it makes sequence usage slower. It depends: do we need every number in the range or can we cope with gaps? As most sequence usage is simply to assign uniqueness gaps do not matter. They do matter in some circumstances, e.g. in an audit subsystem.
The LAST_NUMBER column on USER_SEQUENCES shows the last number assigned. But, as the example below shows, beware. Before we have called the sequence in this session, LAST_NUMBER shows the actual last number given by the sequence. Once the sequence has satisfied a NEXTVAL call the LAST_NUMBER column shows the last number in the current cache. We can see the actual last number assigned with a CURRVAL statement.
Regards, APC
P.S. The rows in your block are being displayed in the "wrong" order because your block does not have an ORDER BY property to sort rows by ID.
SQL> CREATE SEQUENCE test_seq;
Sequence created.
SQL> SELECT last_number, cache_size
  2  FROM   user_sequences
  3  WHERE  sequence_name = 'TEST_SEQ';
LAST_NUMBER CACHE_SIZE                                                         
          1         20                                                         
SQL> SELECT test_seq.NEXTVAL FROM dual;
   NEXTVAL                                                                     
         1                                                                     
SQL> SELECT last_number, cache_size
  2  FROM   user_sequences
  3  WHERE  sequence_name = 'TEST_SEQ';
LAST_NUMBER CACHE_SIZE                                                         
         21         20                                                         
SQL> SELECT test_seq.NEXTVAL FROM dual;
   NEXTVAL                                                                     
         2                                                                     
SQL> SELECT last_number, cache_size
  2  FROM   user_sequences
  3  WHERE  sequence_name = 'TEST_SEQ';
LAST_NUMBER CACHE_SIZE                                                         
         21         20                                                         
SQL> SELECT test_seq.NEXTVAL FROM dual;
   NEXTVAL                                                                     
         3                                                                     
SQL> ROOLBACK;
Rollback complete.
SQL> SELECT last_number, cache_size
  2  FROM   user_sequences
  3  WHERE  sequence_name = 'TEST_SEQ';
LAST_NUMBER CACHE_SIZE                                                         
         21         20                                                         
SQL> SELECT test_seq.NEXTVAL FROM dual;
   NEXTVAL
         4
SQL> SELECT last_number, cache_size
  2  FROM   user_sequences
  3  WHERE  sequence_name = 'TEST_SEQ';
LAST_NUMBER CACHE_SIZE
         21         20
SQL> SELECT test_seq.CURRVAL FROM dual;
   NEXTVAL
         4
SQL>