问题 问答题


事务T1,T2,T3分别对数据D1,D2,D3并发操作如下图所示,其中T1与T2问的并发操作存在的问题是 (1) 。T2与T3问的并发操作存在的问题是 (2)

时间T1T2T3
t1读D1=50
t2读D2=100
t3读D3=300
t4x1=D1+D2+D3
t5读D2=100
t6读D3=300
t7读D2=100
t8D2=D3-D2
t9写D2
t10读D1=50
t11读D2=200
t12读D3=300
t13x1=D1+D2+D3
t14验证不对D2=D2+50
t15写D2

答案

参考答案:丢失修改

解析: 本题考查数据库并发控制方面的基础知识应用。所谓的并发操作,是指在多用户共享的系统中,许多用户可能同时对同一数据进行操作。并发操作带来的问题是数据的不一致性,主要有丢失更新、不可重复读和读脏数据3类,主要的原因是事务的并发操作破环了事务的隔离性。

单项选择题

Large parts of the world have not enjoyed the remarkable global progress in health conditions that have taken place over the past century. Indeed, millions of deaths in impoverished nations are avoidable with prevention and treatment options that the rich world already uses. This year, 10 million children will die in low and middle income countries. If child death rates were the same as those in developed countries this figure would be lower than 1 million. Conversely, if child death rates were those of rich countries just 100 years ago, the figure would be 30 million. Today’s tools for improving health are so powerful and inexpensive that health conditions could be reasonably good even in poor countries if policy makers spent even relatively little in the right places.

Recent research for the Copenhagen Consensus identifies several highly cost-effective options that would tackle some of the planet’s most urgent health problems. The most promising investment is in tuberculosis treatment. Some 90 percent of the 1.6 million tuberculosis deaths in 2003 occurred in low-and middle-income countries. Because tuberculosis affects working-age people, it can be a trigger of household poverty. The cornerstone of control is prompt treatment using first-line drugs, which doesn’t require a sophisticated health system. Spending $1 billion on tuberculosis treatment in a year would save 1 million lives. Because good health accompanies higher levels of national economic welfare in the long run, the economic benefits are worth $ 30 billion.

The second most cost-effective investment is tackling heart disease. Heart disease might not seem like a pressing issue for poor nations, but it represents more than a quarter of their death toll. Measures to reduce risk factors other than smoking — high intake or saturated animal fat, obesity, binge drinking of. alcohol, physical inactivity, and low fruit and vegetable consumption — have had little success. Treating acute heart attacks with inexpensive drugs is, however, cost-effective. Spending $ 200 million could avert several hundred thousand deaths, yielding benefits that are 25 times higher than costs.

The third option is prevention and treatment of malaria. A billion dollars would expand the provision of insecticide-treated bed-nets and facilitate provision of highly effective treatment. This would save more than a million child deaths and produce economic benefits worth $ 20 billion.

The fourth alternative for policymakers is to focus on child health initiatives. The best measures are familiar ones expanding immunization coverage, promoting breastfeeding, increasing the use of simple and cheap treatments for diarrhea and childhood pneumonia, and so on.

Even if the costs of all these initiatives were two or three times higher than we estimate, these efforts would still provide amazing opportunities to reduce health inequality and do good in the world.

What would be the best title for this passage?()

A.Best Options for Tackling World’s Killer Diseases.

B.Cost-effective Investment in Impoverished Nations.

C.Health Inequality between Developed and Developing Countries.

D.Earth’s Killer Diseases: Tuberculosis, Heart Attack and Malaria.

单项选择题