A rule of thumb for choosing indexes for a relational database includes which of the following?

[A].	Indexes are more useful on smaller tables.
[B].	Indexes are more useful for columns that do not appear frequently in the WHERE clause in queries.
[C].	Do not specify a unique index for the primary key of each table.
[D].	Be careful indexing attributes that have null values.

Answer: Option D

Explanation:

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