Hi Ajay,
a 'select count(*)' would look into the database filedirectory and return the result immediately, if there are valid file directory counters for that table (which should be the case for every table, unless there is a problem).
If, however, no filedirectory counters are found, the database would need to perform a full table scan reading all 950 GB to count every single row and return the total. That is the reason for the long runtime of a select count(*) in your system... Again, easiest way would be to run a check data in admin mode, if you can schedule a downtime for the affected production system.
Thorsten