fragment by date expression

Hi, in one of our table 'dta' , the old records are not physically deleted , we have a field 'dtadte' that might content the date of the suppression. This table is now growing very fast, and I want to cut the number of rows to scan by fragmenting the table in the following way: ALTER FRAGMENT ON TABLE dta INIT FRAGMENT BY EXPRESSION dtadte is null IN db1dbs, remainder IN db2dbs; When I try a select like SELECT * FROM dta WHERE dtadte is null, it always scans both fragments. If I fragment using ALTER FRAGMENT ON TABLE dta INIT FRAGMENT BY EXPRESSION dtadte >'01/01/1900' IN db1dbs, remainder IN db2dbs; When I try a select like SELECT * FROM dta WHERE dtadte = '01/03/1900', it scans only one of the fragment, if I use TODAY in the where clause it scans both fragments again. It's not very clear to me, any help would be appreciated. VLG.