Indexes and Nulls (198)

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.

索引與空值
1. 前面這段每看懂 , 哪位大哥看到了 , 明白的話賜教一下.
2. Oracle 不會將索引列全部為 NULL 的資料行加入到索引中.不過點陣圖索引是個例外,簇鍵的列值為 NULL 時也是例外

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