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Consider the query: Mysql＞ SET @run = 15; Mysql＞ EXPLAIN SELECT objective, stage, COUNT (stage) FROM iteminformation WHERE run=@run AND objective=’7.1’ GROUP BY objective,stage ORDER BY stage; The iteminformation table has the following indexes; Mysql＞ SHOW INDEXES FROM iteminformation: This query is run several times in an application with different values in the WHERE clause in a growing data set. What is the primary improvement that can be made for this scenario?（）
A:Do not pass a user variable in the WHERE clause because it limits the ability of the optimizer to use indexes
B:Add an index on the objective column so that is can be used in both the WHERE and GROUP BY operations
C:Drop the run_2 index because it has caused a conflict in the choice of key for this query
D:Execute the run_2 index because it has caused a conflict in the choice of key for this query
E:Add a composite index on (run,objective,stage) to allow the query to fully utilize an index

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出自：Oracle认证

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