Cost-based optimization in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the main goal of cost-based optimization

What is the primary objective of cost-based optimization in a database management system?

A To store query results permanently for faster future access

B To execute queries as quickly as possible without considering resource usage

C To randomly select a query plan to balance load across servers

D To find the query execution plan with the lowest estimated resource usage

💡 Hint

Think about what 'cost' refers to in query execution.

Cost-based optimization aims to choose the query plan that uses the least resources like CPU, memory, and disk I/O, leading to efficient query execution.

📋 Factual

intermediate

Components considered in cost estimation

Which of the following components is NOT typically considered by a cost-based optimizer when estimating the cost of a query plan?

A CPU usage required to process the query

B Disk I/O operations needed to read data

C Network latency between client and server

D Memory usage during query execution

💡 Hint

Consider what the optimizer controls directly during query execution.

Cost-based optimizers focus on resources used by the database engine itself, such as CPU, disk I/O, and memory. Network latency is external and usually not part of cost estimation.

🔍 Analysis

advanced

Analyzing the effect of statistics on cost-based optimization

How do outdated or inaccurate statistics affect the decisions made by a cost-based optimizer?

A They can cause the optimizer to choose inefficient query plans leading to slower execution

B They have no effect since the optimizer uses fixed rules regardless of statistics

C They cause the optimizer to skip cost estimation and use default plans

D They improve optimization by forcing the optimizer to try multiple plans

💡 Hint

Think about how the optimizer estimates costs based on data distribution.

The optimizer relies on statistics about data distribution to estimate costs accurately. If these statistics are outdated, it may misestimate costs and pick suboptimal plans.

❓ Comparison

advanced

Comparing cost-based and rule-based optimization

Which statement best describes a key difference between cost-based and rule-based query optimization?

A Cost-based optimization uses estimated resource costs to choose plans; rule-based uses fixed priority rules without cost estimation

B Rule-based optimization always produces faster query plans than cost-based optimization

C Cost-based optimization ignores data statistics while rule-based relies on them heavily

D Rule-based optimization is more flexible and adapts to changing data distributions automatically

💡 Hint

Consider how each optimizer decides which plan to use.

Cost-based optimizers estimate resource usage costs to pick the best plan, while rule-based optimizers follow fixed rules without estimating costs.

❓ Reasoning

expert

Identifying the impact of join order on query cost

Given a query joining three tables A, B, and C, why does the order in which tables are joined affect the cost estimated by a cost-based optimizer?

A Because the optimizer only supports joining tables in alphabetical order

B Because different join orders can change the size of intermediate results, affecting CPU and I/O costs

C Because join order does not affect cost; all orders produce the same result and cost

D Because the database engine executes joins in parallel regardless of order

💡 Hint

Think about how intermediate data size influences resource use.

Joining tables in different orders can produce larger or smaller intermediate results, which changes the amount of CPU and disk I/O needed, thus affecting the total cost.