Discussion
A rule of thumb for choosing indexes for a relational database includes which of the following?
Answer: D
Correct answer: Be careful indexing attributes that have null values.
Nulls complicate an index, since how they are stored and whether they can be searched for varies between products, and a column with many of them may not benefit from indexing at all. The other three rules are each stated backwards: indexes help most on large tables, on columns that do appear in WHERE clauses, and a unique index on the primary key is essential rather than avoidable.
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