第59題-OCP認證062考試題庫2019最新考題解析
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我要咨詢2018年的時候，OCP 11g考試題庫大更新，052 053出現了很多新題，
2019年，11g即將停考的前期，Oracle又出現了一次大變動，12c出現了很多新題，比如062 063題庫大更新
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QUESTION 59
Examine the parameter for your database instance:
NAME TYPE VALUE
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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：題目的意思是執行前用 explain plan 產生了執行計劃，然后發現真正執行時
執行計劃和之前產生的不一樣，關鍵的地方是該語句是第一次執行，所以執行不一樣是由于做了
動態采樣。但是如果該語句執行了 2 次的話，如果執行計劃不一樣，那么正確的答案就是 C 了，
因為 12c 以后優化器就會用到基數反饋技術來重新產生執行計劃。)
