"We’re using a
SELECT COUNT(*) FROM Tquery (or something similar) and we have a B*Tree index on table T. However, the optimizer is full scanning the table, rather than counting the (much smaller) index entries. In this case, the index is probably on a set of columns that can contain Nulls. Since a totally Null index entry would never be made, the count of rows in the index will not be the count of rows in the table. Here the optimizer is doing the right thing—it would get the wrong answer if it used the index to count rows."
create index t_table_col on t(col, 0);