Question

I have a table with a PK that grows fairly quickly, but since rows are fairly consistently deleted, it becomes a very sparse table quickly as such:

   ID     VALUE
 ----------------
   1     'Test'
   5     'Test 2'
   24    'Test 3'
   67    'Test 4'

Is there a way that I can automatically insert the next value in the missing IDs so that I don’t grow that ID extremely large? For example, I’d like to insert ‘Test 5’ with ID 2.

Answer 1

I deleted my answer about identity since they are not involved. It would be interesting to see if you are using this as a clustered index key, since to fill in gaps would violate the rule of thumb of strictly increasing values.

To just fill in gaps is relatively simple with a self-join and since you have a primary key, this query should run quickly to find the first gap (but of course, how are you handling simultaneous inserts and locks?):

SELECT lhs.ID + 1 AS firstgap
FROM tablename AS lhs
LEFT JOIN tablename AS rhs
    ON rhs.ID = lhs.ID + 1
WHERE rhs.ID IS NULL

And inserting batches of records requires each insert to be done separately, while IDENTITY can handle that for you…