Snapshot Isolation (SI) is a multiversion concurrency control algorithm, first described in Berenson et al. [1995]. SI is attractive because it provides an isolation level that avoids many of the common concurrency anomalies, and has been implemented by Oracle and Microsoft SQL Server (with certain minor variations). SI does not guarantee serializability in all cases, but the TPC-C benchmark application [TPC-C], for example, executes under SI without serialization anomalies. All major database system products are delivered with default nonserializable isolation levels, often ones that encounter serialization anomalies more commonly than SI, and we suspect that numerous isolation errors occur each day at many large sites because of this, leading to corrupt data sometimes noted in data warehouse applications. The classical justification for lower isolation levels is that applications can be run under such levels to improve efficiency when they can be shown not to result in serious errors, but little or no guidance has been offered to application programmers and DBAs by vendors as to how to avoid such errors. This article develops a theory that characterizes when nonserializable executions of applications can occur under SI. Near the end of the article, we apply this theory to demonstrate that the TPC-C benchmark application has no serialization anomalies under SI, and then discuss how this demonstration can be generalized to other applications. We also present a discussion on how to modify the program logic of applications that are nonserializable under SI so that serializability will be guaranteed.
SOTERIADES, ELPIDOFOROS S., RUSS HAUSER, ICHIRO KAWACHI, DIMITRIOS LIAROKAPIS, DAVIDC. CHRISTIANI, AND STEFANOS N. KALES. Obesity and cardiovascular disease risk factors in firefighters: a prospective cohort study. Obes Res. 2005;13:1756 -1763. Objective: Obesity, despite being a significant determinant of fitness for duty, is reaching epidemic levels in the workplace. Firefighters' fitness is important to their health and to public safety. Research Methods and Procedures:We examined the distribution of BMI and its association with major cardiovascular disease (CVD) risk factors in Massachusetts firefighters who underwent baseline (1996) and annual medical examinations through a statewide medical surveillance program over 5 years of follow-up. We also evaluated firefighters' weight change over time. Results: The mean BMI among 332 firefighters increased from 29 at baseline to 30 at the follow-up examination (2001), and the prevalence of obesity increased from 35% to 40%, respectively (p Ͻ 0.0001). In addition, the proportion of firefighters with extreme obesity increased 4-fold at follow-up (from 0.6% to 2.4%, p Ͻ 0.0001). Obese firefighters were more likely to have hypertension (p ϭ 0.03) and low high-density lipoprotein-cholesterol (p ϭ 0.01) at follow-up. Firefighters with extreme obesity had an average of 2.1 CVD risk factors (excluding obesity) in contrast to 1.5 CVD risk factors for normal-weight firefighters (p ϭ 0.02). Finally, on average, normal-weight firefighters gained 1.1 pounds, whereas firefighters with BMI Ն 35 gained 1.9 pounds per year of active duty over 5 years of follow-up. Discussion: Obesity is a major concern among firefighters and shows worsening trends over time. Periodic medical evaluations coupled with exercise and dietary guidelines are needed to address this problem, which threatens firefighters' health and may jeopardize public safety.
The authors evaluated blood pressure and antihypertensive medication use in 334 firefighters in an occupational medical surveillance program. Firefighters received written summaries of their examination results, including blood pressures, and were encouraged to see their personal physicians for any abnormal results. The mean age of the participants was 39 years, and the vast majority were men (n=330). The prevalence of hypertension was 20% at baseline (1996), 23% in 1998, and 23% in 2000. Among firefighters with high blood pressure readings, only 17%, 25%, and 22% were taking antihypertensive medications at the baseline, 1998, and 2000 examinations, respectively. Medical surveillance was effective in detecting hypertension in firefighters; however, after 4 years of follow-up, only 42% of hypertensives were receiving treatment with medications, including only 22% of firefighters with hypertensive readings. Overall, 74% of hypertensives were not adequately controlled. Possible reasons for low treatment rates may be the inadequate recognition among primary care physicians that mild hypertension is a significant risk factor for cardiovascular disease.
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