[sql] Best way to reset an Oracle sequence to the next value in an existing column?

For some reason, people in the past have inserted data without using sequence.NEXTVAL. So when I go to use sequence.NEXTVAL in order to populate a table, I get a PK violation, since that number is already in use in the table.

How can I update the next value so that it is usable? Right now, I'm just inserting over and over until it's successful (INSERT INTO tbl (pk) VALUES (sequence.NEXTVAL)), and that syncs up the nextval.

Today, in Oracle 12c or newer, you probably have the column defined as GENERATED ... AS IDENTITY, and Oracle takes care of the sequence itself.

You can use an ALTER TABLE Statement to modify "START WITH" of the identity.

ALTER TABLE tbl MODIFY ("ID" NUMBER(13,0) GENERATED BY DEFAULT ON NULL AS IDENTITY MINVALUE 1 MAXVALUE 9999999999999999999999999999 INCREMENT BY 1 START WITH 3580 NOT NULL ENABLE);

With oracle 10.2g:

select  level, sequence.NEXTVAL
from  dual 
connect by level <= (select max(pk) from tbl);

will set the current sequence value to the max(pk) of your table (i.e. the next call to NEXTVAL will give you the right result); if you use Toad, press F5 to run the statement, not F9, which pages the output (thus stopping the increment after, usually, 500 rows). Good side: this solution is only DML, not DDL. Only SQL and no PL-SQL. Bad side : this solution prints max(pk) rows of output, i.e. is usually slower than the ALTER SEQUENCE solution.

In my case I have a sequence called PS_LOG_SEQ which had a LAST_NUMBER = 3920.

I then imported some data from PROD to my local machine and inserted into the PS_LOG table. Production data had more than 20000 rows with the latest LOG_ID (primary key) being 20070. After importing I tried to insert new rows in this table but when saving I got an exception like this one:

ORA-00001: unique constraint (LOG.PS_LOG_PK) violated

Surely this has to do with the Sequence PS_LOG_SEQ associated with the PS_LOG table. The LAST_NUMBER was colliding with data I imported which had already used the next ID value from the PS_LOG_SEQ.

To solve that I used this command to update the sequence to the latest \ max(LOG_ID) + 1:

alter sequence PS_LOG_SEQ restart start with 20071;

This command reset the LAST_NUMBER value and I could then insert new rows into the table. No more collision. :)

Note: this alter sequence command is new in Oracle 12c.

Note: this blog post documents the ALTER SEQUENCE RESTART option does exist, but as of 18c, is not documented. Its apparently intended for internal Oracle use.

You can temporarily increase the cache size and do one dummy select and then reset the cache size back to 1. So for example

ALTER SEQUENCE mysequence INCREMENT BY 100;

select mysequence.nextval from dual;

ALTER SEQUENCE mysequence INCREMENT BY 1;

If you can count on having a period of time where the table is in a stable state with no new inserts going on, this should do it (untested):

DECLARE
  last_used  NUMBER;
  curr_seq   NUMBER;
BEGIN
  SELECT MAX(pk_val) INTO last_used FROM your_table;

  LOOP
    SELECT your_seq.NEXTVAL INTO curr_seq FROM dual;
    IF curr_seq >= last_used THEN EXIT;
    END IF;
  END LOOP;
END;

This enables you to get the sequence back in sync with the table, without dropping/recreating/re-granting the sequence. It also uses no DDL, so no implicit commits are performed. Of course, you're going to have to hunt down and slap the folks who insist on not using the sequence to populate the column...