第59题-OCP认证062考试题库2019最新考题解析
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我要咨询2018年的时候，OCP 11g考试题库大更新，052 053出现了很多新题，
2019年，11g即将停考的前期，Oracle又出现了一次大变动，12c出现了很多新题，比如062 063题库大更新
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QUESTION 59
Examine the parameter for your database instance:
NAME TYPE VALUE
----------------------------------------------
optimizer_adaptive_reporting_only boolean FALSE
optimizer_capture_sql_baselines boolean FALSE
optimizer_dynamic_sampling integer 2
optimizer_features_enable string 12.1.0.1
You generated the execution plan for the following query in t he plan table and noticed that the nested loop join was done. After actual execution of the query, you notice that the hash join was done in the execution plan:
dentify the reason why the optimizer chose different execution plans.
SQL> SELECT product_name
FROM order_item o,product_information p
WHERE o.unit_price=15
AND quantity>1
AND p.product_id=o.product_id;
30 rows selected.
A. The optimizer used a dynamic plan for the query.
B. The optimizer chose different plans because automatic dynamic sampling was enabled.
C. The optimizer used re-optimization cardinality feeDBAck for the query.
D. The optimizer chose different plan because extended statistics were created for the columns used.
Correct Answer: B
Section: (none)
Explanation(解析：题目的意思是执行前用 explain plan 产生了执行计划，然后发现真正执行时
执行计划和之前产生的不一样，关键的地方是该语句是第一次执行，所以执行不一样是由于做了
动态采样。但是如果该语句执行了 2 次的话，如果执行计划不一样，那么正确的答案就是 C 了，
因为 12c 以后优化器就会用到基数反馈技术来重新产生执行计划。)
