In case you're interested in solving this problem in SQL Server, this is a cool solution
http://thefirstsql.com/2011/07/08/how-to-find-gaps-in-identity-columns-at-the-speed-of-light/

>>A table has an index column that starts at one and goes to N, with some missing values. Is there an SQL Select statement (its ok to use more than one) that will return the first x items that are NOT present in the column?
>>
>>TIA,
>>
>>Alex
>
>Create a Numbers table (see, for example):
>http://blogs.lessthandot.com/index.php/DataMgmt/DataDesign/passing-multiple-ranges-to-stored-proced
>
>Once you have the Numbers table:
>

>select top (N) N.Number from myTable T RIGHT JOIN Numbers N on T.IndexCol = N.Number
>WHERE T.IndexCol IS NULL
>ORDER BY N.Number