在关系R(A1, A2, A3)和S(A2, A3, A4)上进行关系运算的4个等价的表达式E1、E2、E3、E4，如下所示：
如果严格按照表达式运算顺序执行，则查询效率最高的是表达式（ ）。
- A.E1
- B.E2
- C.E3
- D.E4
正确答案及解析
正确答案
解析
本题考查关系代数运行效率问题，解决这类问题有一个通用原则，即存在连接操作与选择操作的尽可能早地完成选择操作。这样能降低参与连接操作的数据量，极大提升运算效率。
包含此试题的试卷
你可能感兴趣的试题
Advancements in （ ）have contributed to the growth of the automotive industry through the creation and evolution of self-driving vehicles.
-
- A.Artificial Intelligence
- B.Cloud Computing
- C.Internet of Things
- D.Big Data
- 查看答案
In project human resource management , （ ）is not a source of power for the project manager.
-
- A.referent power
- B.expert power
- C.reward power
- D.audit power
- 查看答案
At the project establishment stage , the feasibility study mainly includes techinical feasibility analysis , （ ）, operation environment feasibility analysis and other aspects of feasibility analysis.
-
- A.detail feasibility analysis
- B.opportunity analysis
- C.economic feasibility analysis
- D.risk analysis
- 查看答案
（ ）is a grid that shows the project resources assigned to each work package.
-
- A.Stakeholder engagement assessment matrix
- B.Requirements traceability matrix
- C.Probability and impact matrix
- D.Responsibility assignment matrix
- 查看答案
Xinhua News Agency reported in January 2022，Chian will further promote the developmet of a digital economy during the 14th Five-Year Plan eriod（2021-2025）. The plan also emphasized industrial （ ）transformation.
-
- A.digital
- B.networking
- C.intelligentize
- D.informatization
- 查看答案