index selectivity

Hi,

does anybody know how exactly the 'compute statistics' works? If you have a table with an index on col1 and the table has 1000 entries and 1000 different entries in col1, you have a selectivity of 1/1000 i.e. the index will be used. If you have 500 entries with value1 and 500 entries with value2, the selectivity is 1/2 and the index will not be used. What matters if you have 999 entries with value1 and 1 value with value2? Does the selectivity remain 1/2 and the index will NEVER be used or is the optimizer so clever to know that the selectivity of value1 is 1 (no use of index) and the selectivity of value2 is 1/1000 (use index)?

Thanks
Franz Received on Tue Oct 19 1999 - 09:55:44 CDT