问题描述：
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Consider the query:考虑一下这个问题：This query is run several times in an application with different values in the WHERE clause in a growing data set．此查询在一个应用程序中多次运行，其中WHERE子句中的值在不断增长的数据集中。What is the primary improvement that can be made for this scenario？对于这种情况，可以做的主要改进是什么？
A．Execute the run_2 index because it has caused a conflict in the choice of key for this query.
B．Drop the run_2 index because it has caused a conflict in the choice of key for this query.
C．Do not pass a user variable in the WHERE clause because it limits the ability of the optimizer to useindexes.
D．Add an index on the objective column so that is can be used in both the WHERE and GROUP BY operations.
E．Add a composite index on (run,objective,stage) to allow the query to fully utilize an index.在(run、objective、stage)上添加一个复合索引，以允许查询充分利用索引。
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