Now, if the field SOMEFIELD declared as NOT NULL, when using conditions WHERE SOMEFIELD IS NULL / NOT NULL, will be scanned the entire table (NATURAL) or used index scan. But, obviously, that:
1. Provided WHERE SOMEFIELD IS NULL, will selected 0 records.
2. Provided WHERE SOMEFIELD IS NOT NULL, will selected all records.
Need using this info at creating the execution plan and at executing query.