Oracle Technology: Impact of indexes and null values on hints and execution plans

Because the B*tree index does not store null values, some Oracle queries do not use indexes when the indexed field is allowed to be empty.

Most of the time, it appears that we can use a full index scan, which may give Oracle the chance to discard the index because of the existence of the null value (Scan).

In this case, the index is not used even with hints,oracle, because of the existence of null values.

Let's look at the following tests.

The Index hints can take effect when the username field is not null.

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Oracle discards this index when the index field is allowed to be null:

When the field is not NULL, the index can be forced to use:

This is the effect of null values on indexes and queries.

Author: 51cto Oracle small-mixed son