基于成本的数据库查询优化浅析

主要介绍基于成本的数据库查询优化的一些基本概念,针对多表连接的三种方法：嵌套循环链接、归并连接和混合连接进行分析和阐述．并成本估算,列出估算公式,     

DIR-FCEV powered by different fuels – Part I: Well-to-wheel analysis for the Brazilian and Spanish contexts

This article describes the life cycle assessment of the Direct Internal Reforming Fuel Cell Electric Vehicle (DIR-FCEV) for the Brazilian and Spanish contexts. This recently proposed vehicle technology produces hydrogen onboard via heat recovery, while the Solid Oxide Fuel Cell (SOFC) and the gas turbine provide electricity to the electric motor with high overall efficiency. The exhaust gas emission was obtained via GASEQ software and the well-to-tank data were collected through literature search. These values were entered in the GREET Model software version v1.3.0.13656 to assess the negative environmental impacts of the DIR-FCEV in terms of global warming potential (GWP) and human toxicity potential (HTP). Using 1 km distance driven by a light-duty vehicle as the functional unit, the tank-to-wheel results indicate that regardless of the fuel, the DIR-FCEV achieved better than the internal combustion engine vehicle fuelled with gasoline A in both categories. Also, the DIR-FCEV powered by biomethane, gasoline A, gasoline C (73% gasoline A and 27% ethanol, in volume basis), glycerine, and ethanol attended all future tailpipe emissions standards stipulated by United States and European Union. Biomethane and gasoline-fuelled DIR-FCEV had more commendatory environmental performance in both countries. Thus, it is expected to obtain environmental indicators to stimulate the use of biofuels synergistically with the advancement of electromobility in Brazil and Spain.     

LNG: An eco-friendly cryogenic fuel for sustainable development

As the demand of natural gas has sharply increased in the last two decades at the global level, the transportation of natural gas from different parts (gas producing to the consuming areas) of the world has become more significant. Liquefaction of natural gas provides a safer and economical alternative for transportation and also increases its storage capabilities. The liquefaction process requires the natural gas to be cooled using various methods of cryogenic processes and also be depressurized to atmospheric conditions for easier and safer storage. LNG transported in cryogenic vessels offers several advantages over pipeline transport of natural gas especially when the gas consuming areas are far away from the gas producing areas. Moreover, LNG as an automobile fuel has a definite edge over other fuels.     

基于丛生树的片段式并行查询优化技术研究

该文以丛生树模型为基础,提出了一种片段式查询执行计划。该执行计划将查询树划分成多个按流水线方式执行的片段,各片段依次执行。该执行计划可以减少中间结果的Ｉ／０次数,更充分地利用内存资源。文中还举例说明了计划的执行过程。     

大牛地气田山1段水平井PDC钻头研究与应用

大牛地气田山西组山1段水平井机械钻速低、钻井周期长,单只钻头寿命短、进尺少,不能满足优快钻井需要。为此,研制了新型双排切削齿P5235MJ型PDC钻头。以储层地质特征为基础,山1段岩心微钻头试验结果为依据,结合现场钻头使用情况,对钻头的冠部轮廓、布齿和水力结构进行优化。现场应用结果表明,该类型钻头对山1段地层适应性较强,与其他型号钻头相比,平均进尺提高44.91%,平均机械钻速提高34.52%。     

MJ螺纹CAD设计

MJ螺纹设计依靠传统的方法只能采用查表的方式进行计算,然后根据参数绘制牙型图,设计效率低。利用计算机辅助设计,输入相关参数,快速而准确地计算出相关尺寸,极大地提高了螺纹的设计精度和效率。     

Rice husk co-firing with coal in a short-combustion-chamber fluidized-bed combustor (SFBC)

This study encompassed the characteristics and performance of co-firing rice husk, a by-product of rice-milling process, with coal in a short-combustion-chamber fluidized-bed combustor (SFBC). Bed phenomena investigated in a cold-flow model combustor showed that with the different mixes of materials, the anticipated offshoot of combustion, the minimum fluidizing velocity (U_mf) was 0.4-0.8 m/s. In concord with axial temperature profiles, axial gas concentration profiles implied that a recirculating ring was able to circumscribe CO within the short-main chamber. The formation, decomposition, and eventual maturity of NO_x characterized the NO_x evolution, inferred from concentration profiles. The impacts of fluidizing velocity and blending ratio on gas emissions and combustion efficiency (E_c) are described. The fluidizing velocity had consequential effect on gas emissions, except NO_x. Surprisingly, NO_x did not hinge much on increased N-content of the mixtures with coal. As expected, increased SO₂ was relevant to increased coal mass. Increased fluidizing velocity adversely affected E_c while increased coal fraction enhanced E_c, mostly >97%.     

DSDHA

Less than 10 years old, DSDHA (Deborah Saunt David Hills Architects) has worked its way up the ladder of building types from nurseries, through to schools and now their first university building opened in summer 2007 in Cambridge. Helen Castle went to visit the studio in Kennington, south London, and discovered a practice that thrives on the heady mixture that educational buildings offer, combining social engagement with complex client needs. Copyright © 2007 John Wiley & Sons, Ltd.     

MJ内螺纹横截面形状及定中心方法的研究

叙述了应用于航空航天产品中的ＭＪ内螺纹轴截面不同位置上径向距离的特点， 详细分析了ＭＪ内螺纹横截面图形的形状，并提出了一种利用ＣＣＤ测量系统对具有特殊内 表面形状的ＭＪ内螺纹进行参数检测时的定中心方法，为其量化检测提供基础理论上的依据。     

基于成本的数据库查询优化浅析

主要介绍基于成本的数据库查询优化的一些基本概念,针对多表连接的三种方法:嵌套循环链接、归并连接和混合连接进行分析和阐述,并成本估算,列出估算公式。