讲解 DTS207TC Database Development and Design讲解 Python编程

Database Development and Design (DTS207TC)

Assessment 002: Individual Coursework

Weight: 40%

Maximum Marks: 100

Overview & Outcomes

This course work will be assessed for the following learning outcomes:

A. Identify and apply the principles underpinning transaction management within DBMS.

E. State the main concepts in data warehousing and data mining.

Submission

You must submit the following files to LMO:

1)A report named as Your_Student_ID.pdf.

2)A directory containing all your source code, named as Your_Student_ID_code.

NOTE: The report shall be in A4 size, size 11 font, and shall not exceed 9 pages in length. You can include only key code snippets in your reports. The complete source code can be placed in the attachment.

Question 6: Storage Management (40 marks)

In a database storage system, the cache hit rate has a significant impact on its performance. Different cache strategies will result in different cache hit ratios. Now, we have recorded 2 datasets (please download from LMO), containing CPU access requests to memory for a period of time. They both have 10,000 items from addresses 0 to 63. We will simulate the process of the CPU reading and caching data from the memory through a program in the table below (also can be download from LMO) . Please run the program to compare the hit rates of different strategies:

Python

import random

from collections import deque

class RandomPolicy:

def __init__(self, size):

self.size = size

self.cache = []

self.name = 'rr'

random.seed(207)

def access(self, current):

if current in self.cache: # hit!

return True

self.cache.append(current)

if len(self.cache) > self.size: # exceed

self.cache.remove(random.choice(self.cache))

return False

class FifoPolicy:

def __init__(self, size):

self.size = size

self.cache = deque()

self.name = 'fifo'

def access(self, current):

if current in self.cache: # hit!

return True

if len(self.cache) == self.size: # full

self.cache.popleft()

self.cache.append(current)

return False

def run_test(trace, pol):

hit = []

for i in range(len(trace)):

# update cache

hit += [pol.access(trace[i])]

return sum(hit) / len(hit)

if __name__ == '__main__':

# parameters

caps = [1, 2, 3, 4, 5]

# load trace from file

traces = []

for name in ['trace1.txt', 'trace2.txt']:

with open(name) as f:

traces += [list(eval(f.read()))]

# test all strategies

strategies = [

FifoPolicy,

RandomPolicy,

]

# run strategy over trace

for i in range(len(traces)):

for cap in caps:

for Strategy in strategies:

pol = Strategy(size=cap)

print(f'data={i +

1},\tcap={cap},\tname={pol.name},\thitrate={run_test(traces[i], pol)}')

print()

You need to analyze the characteristics of this data and analyze why the hit rates of the two strategies are different on the two data sets (20 mark). Design and implement a strategy which can achieve better results than the RandomPolicy strategy on the trace2 data set. Record the hit rates you observed in the table below (with snapshot) (20 marks).

Question 7: Indexing (30 marks)

Consider a hard disk with a sector size of B = 512 bytes. A CUSTOMER file contains approximately r = 40,000 records. Each record includes the following fields: Name (30 bytes), Ssn (9 bytes), Email (30 bytes), Address (50 bytes), Phone (15 bytes), and Birth_date (8 bytes). The Ssn field is the primary key.

The file system uses 4KB blocks for allocation.

(a) Calculate the number of blocks required for an unspanned organization. Then, discuss how the discrepancy between sector size and block size might affect sequential access performance, and whether you would recommend using a different block size for this scenario. (6 marks)

(b) The records are physically ordered on Ssn. Calculate the maximum number of block accesses for a binary search. During system testing, developers notice that batch queries processing large ranges of Ssn values perform. 30% slower than expected when using binary search as the primary lookup method. Provide two possible explanations for your scenario. (6 marks)

(c) A sparse index is built on Ssn. Calculate the number of block accesses to retrieve a record using this index in ideal scenario. During usage, it is found that the performance of the index search continues to decline. Identify two potential reasons why the index performance gain is less than theoretical expectations. (6 marks)

(d) A multi-level primary index is constructed. During the design review, two proposals are made: Proposal A: Use the standard multi-level index structure; Proposal B: Based on Proposal A, select 10 index blocks and cache them persistently in memory. Calculate the number of index levels needed for the Proposal A. Then, compare the two proposals in terms of implementation complexity and computation time under a workload with 10% of the Ssn accounting for 90% of the queries. (6 marks)

(e) A B+ tree index is built with order p = 50. Calculate the maximum number of records a height 4 tree can index. During maintenance, it's observed that, during frequent insertion and deletion operations, the tree height changes frequently between 3 and 4 even with relatively stable data size.

Explain what might be causing this fluctuation and suggest one strategy to stabilize the tree height with optimal performance. (6 marks)

Question 8: Transaction (30 marks)

Consider a database with a relation Account (AccountID, Balance) and initial state:

(a) The following transactions represent a fund transfer (T1) and a real-time balance report (T2) that run concurrently. (10 marks)

The application requirement states that the report must never reflect a financially inconsistent state.

However, under certain database configurations, T2 might output a total of 20 (instead of the correct 120).

(i) Explain under which isolation level(s) this inconsistent total of 20 could occur, and describe the exact sequence of operations in a concurrent schedule that leads to this result.

(ii) The development team proposes using the SERIALIZABLE isolation level to fix this issue. Critically

evaluate this proposal by discussing one key advantage and two potential drawbacks (considering both performance and system complexity) for this specific application scenario.

(b) Now consider these transactions: a process adding a new account (T3), and an audit process

Suppose the application requirement for the audit is that it must have a consistent view of the database throughout its execution.

(i) Is it possible for the two SUM queries in T4 to return different values? Analyze this possibility under at least three different isolation levels, providing a brief concurrent schedule for each case where the results differ.

(ii) During a system design review, an engineer suggests: "We can just use the REPEATABLE READ

isolation level to solve all our concurrency problems in this audit process." Write a brief response  evaluating this suggestion. Your response should consider whether this is sufficient, necessary, and practical for meeting the audit requirement.

(c) Consider the following observation from production logs: (10 marks)

l T5 (Data Maintenance): Inserts 100 new account records in a single transaction.

l T6 (Analytics Query): Runs SELECT COUNT(*) FROM Account; twice within its transaction and gets two different results.

The team initially diagnosed this as a "phantom read."

(i) Under which isolation level(s) is this phenomenon possible?

(ii) A DBA comments: "While this looks like a phantom read, the actual impact and the appropriate fix might be different if those 100 new accounts were all inserted with a zero balance." Briefly explain the DBA's reasoning. Why might the business impact and the technical solution be different if the new accounts have a zero balance, even though the phenomenon looks the same?