NULL values in indexes are considered to be distinct except when all the non-NULL
values in two or more rows of an index are identical, in which case the rows are
considered to be identical. Therefore, UNIQUE indexes prevent rows containing NULL
values from being treated as identical. This does not apply if there are no non-NULL
values—in other words, if the rows are entirely NULL.

Oracle does not index table rows in which all key columns are NULL, except in the case
of bitmap indexes or when the cluster key column value is NULL.

索引与空值
1. 前面这段每看懂 , 哪位大哥看到了 , 明白的话赐教一下.
2. Oracle 不会将索引列全部为 NULL 的数据行加入到索引中.不过位图索引是个例外,簇键的列值为 NULL 时也是例外