unique index allows the field to be null, but does the null value of the field invalidate the index? What"s the point of having a unique index with null values?
unique index allows the field to be null, but does the null value of the field invalidate the index? What"s the point of having a unique index with null values?
your understanding of index invalidation is not quite right, for example, column an is indexed, so when the an is null condition is applied to this column, the index is not used (it does not mean that the index is invalid), but when you use the index.' You can use the index.
to put it simply: single-column indexes do not store null values, and composite indexes do not store all null values
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