數(shù)據(jù)庫復(fù)習(xí)題2(答案)-資料下載頁
2025-06-26 06:07本頁面
【正文】 xecuted before Query 2, and both queries are executed using a file scan of the enrolled table.Which replacement policy would you remend for the buffer manager to use to support this workload?(b) Now assume that we have retrieved the snum’s of students who have taken ‘Database Systems’ from the enrolled table. In the exact order of the retrieved snum’s (not necessarily in sorted order),we then retrieve the names of those students via repeated lookups in the B+ Tree on snum.For these repeated accesses to the index on , which replacement policy would you remend for efficient buffer management?Query 1: select snum from student s, enrolled e where = and ame like ‘Database Systems’。Query 2: select snum from student s, enrolled e where = and ame like ‘Operating Systems’。Answer: (a) Not LRU. MRU or LRU2.(b) Now we have repeated equality searches over the B+ Tree on sid, with no duplicate values in the search key (because the schema does not allow a student to take the same course twice). Two possible answers: The B+ Tree pages close to the top are repeated accessed but those at the leaf level are rarely reused. So we can use LRU. LIFO (Last In First Out) is another possible answer. This is because over time, we have cached most nodes close to the top in memory. So the nodes recently read from the disk are mostly close to the leaves. So LIFO will replace those leaf or closetoleaf nodes to make room for the newly requested nodes. We also accept other answers if the student can justify well.
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