Question

According to Oracle 9i performance tuning tips and techniques, using IS NULL or IS NOT NULL will suppress the index of the column – if there is an index on the column, and recommends making the column not null-able and using a DEFAULT.

The question is, will a null-able column suppress an index even if my select query does not use IS NULL or IS NOT NULL explicitly?

Answer 1

For a general query, the fact that a column is nullable won’t prevent the optimizer or query engine from using an index unless the criterion on the column is IS NULL or IS NOT NULL. So, a general WHERE column = somevalue query will use the index if that is appropriate.

The situation where the (nullable) column is one of several columns in the index is less clear. In the absence of concrete information to the contrary, I’d assume that the index will be used if there are criteria on the other columns in the index, possibly even with an IS [NOT] NULL criterion on the nullable column. It might depend on whether or not the nullable column is the lead column for the index.