Its quite that my interpretation of the documented statement may differ.
Here is the document link http://download.oracle.com/docs/cd/E11882_01/server.112/e10595/indexes002.htm#ADMIN11711
"Columns That Are Suitable for Indexing
Some columns are strong candidates for indexing. Columns with one or more of the following characteristics are candidates for indexing:
* Values are relatively unique in the column.
* There is a wide range of values (good for regular indexes).
* There is a small range of values (good for bitmap indexes).
* The column contains many nulls, but queries often select all rows having a value. In this case, use the following phrase:
WHERE COL_X > -9.99 * power(10,125)
Using the preceding phrase is preferable to:
WHERE COL_X IS NOT NULL
This is because the first uses an index on COL_X (assuming that COL_X is a numeric column).
Columns That Are Not Suitable for Indexing
Columns with the following characteristics are less suitable for indexing:
* There are many nulls in the column and you do not search on the not null values.
LONG and LONG RAW columns cannot be indexed."
May 27, 2010 - 7:29 am UTC
if col_x is nullable AND col_x is the only thing in the index, then what they say is true.
if col_x is nullable AND col_x is on the leading edge of the index, AND there is some other NOT NULL attribute in the index, then what they say is not really true.
And note: they say things like "less suitable", not "does not work"
Understand how indexes work and you don't need anyones list, you could write your own.