Cost-based optimization in DBMS Theory - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is cost-based optimization in database management systems?

Cost-based optimization is a method used by database systems to choose the most efficient way to execute a query by estimating the cost of different query plans and selecting the one with the lowest cost.

intermediate

What factors does cost-based optimization consider when estimating the cost of a query plan?

It considers factors like CPU usage, disk I/O, memory usage, and network costs to estimate how expensive a query plan will be to execute.

beginner

Why is cost-based optimization important for database performance?

Because it helps the database system run queries faster and use fewer resources by choosing the best execution plan, improving overall system efficiency.

intermediate

How does cost-based optimization differ from rule-based optimization?

Rule-based optimization uses fixed rules to choose query plans, while cost-based optimization evaluates multiple plans and picks the one with the lowest estimated cost.

intermediate

What role do statistics play in cost-based optimization?

Statistics about data, like table size and index distribution, help the optimizer estimate costs accurately and choose the best query plan.

What does cost-based optimization primarily aim to minimize?

A The number of users accessing the database

B The number of tables in a database

C The size of the database

D The estimated resource cost of executing a query

Cost-based optimization tries to find the query plan with the lowest estimated resource cost.

Which of the following is NOT typically considered in cost estimation?

A User login time

B CPU usage

C Disk I/O

D Memory usage

User login time is unrelated to query execution cost.

What helps the optimizer estimate costs more accurately?

A Number of queries run

B User preferences

C Database statistics

D Network speed only

Database statistics provide data distribution and size information needed for cost estimation.

Which optimization method evaluates multiple query plans before choosing one?

A Cost-based optimization

B Rule-based optimization

C Random optimization

D Manual optimization

Cost-based optimization compares different plans based on estimated costs.

Why might a database use cost-based optimization instead of rule-based?

A Because rule-based is faster always

B To find more efficient query plans by considering actual costs

C To ignore data statistics

D To reduce the number of tables

Cost-based optimization uses cost estimates to find better plans than fixed rules.

Explain what cost-based optimization is and why it is used in databases.

Think about how databases decide the best way to run a query.

Describe the role of statistics in cost-based optimization.

Consider what information the optimizer needs to estimate costs.

Practice

1. What is the main goal of cost-based optimization in a database system?

easy

A. To find the most efficient way to execute a query

B. To store data in the smallest space possible

C. To encrypt data for security

D. To backup the database automatically

Solution

1. Step 1: Understand the purpose of cost-based optimization

   Cost-based optimization evaluates different ways to run a query and estimates their costs.

2. Step 2: Identify the main goal

   The goal is to pick the plan with the lowest cost, meaning the fastest or least resource-heavy execution.

3. Final Answer:

   To find the most efficient way to execute a query -> Option A

4. Quick Check:

   Cost-based optimization = efficient query execution [OK]

Hint: Focus on efficiency and speed of query execution [OK]

Common Mistakes:

• Confusing optimization with data storage
• Thinking it handles security tasks
• Assuming it manages backups

2. Which of the following is a key input used by cost-based optimizers to estimate query costs?

easy

A. User login credentials

B. Data statistics like table size and index availability

C. Network bandwidth

D. Database backup schedules

Solution

1. Step 1: Identify what cost-based optimizers use

   They rely on data statistics such as table size, number of rows, and indexes to estimate costs.

2. Step 2: Match the correct input

   Data statistics directly affect the cost estimation, unlike user credentials or backup schedules.

3. Final Answer:

   Data statistics like table size and index availability -> Option B

4. Quick Check:

   Cost estimation uses data statistics [OK]

Hint: Remember: statistics guide cost estimates, not user info [OK]

Common Mistakes:

• Confusing user data with statistics
• Thinking network or backups affect cost estimation
• Ignoring the role of indexes

3. Consider a database query optimizer that chooses between two plans: Plan A costs 50 units, Plan B costs 80 units. Which plan will the optimizer select?

medium

A. Neither plan because cost is ignored

B. Plan B because it has a higher cost

C. Both plans equally because cost does not matter

D. Plan A because it has a lower cost

Solution

1. Step 1: Understand cost comparison

   The optimizer picks the plan with the lowest estimated cost to improve performance.

2. Step 2: Compare given costs

   Plan A costs 50 units, which is less than Plan B's 80 units, so Plan A is preferred.

3. Final Answer:

   Plan A because it has a lower cost -> Option D

4. Quick Check:

   Lower cost plan chosen = Plan A [OK]

Hint: Choose the plan with the smallest cost number [OK]

Common Mistakes:

• Picking higher cost plan mistakenly
• Ignoring cost values
• Assuming cost is irrelevant

4. A cost-based optimizer is not choosing the fastest query plan. What could be a likely reason?

medium

A. The data statistics are outdated or inaccurate

B. The database server is turned off

C. The query syntax is incorrect

D. The user has no permissions

Solution

1. Step 1: Identify factors affecting optimizer decisions

   The optimizer depends on accurate data statistics to estimate costs correctly.

2. Step 2: Analyze the problem cause

   If statistics are outdated, the optimizer may pick a suboptimal plan, causing slower queries.

3. Final Answer:

   The data statistics are outdated or inaccurate -> Option A

4. Quick Check:

   Outdated stats cause wrong plan choice [OK]

Hint: Check if statistics are current to fix optimizer issues [OK]

Common Mistakes:

• Blaming server status without checking stats
• Confusing syntax errors with optimization issues
• Assuming permissions affect plan choice

5. A database has two indexes on a table: one on column A and another on column B. A query filters on both columns. How does cost-based optimization decide which index to use?

hard

A. It ignores indexes and does a full table scan

B. It always uses the index on column A by default

C. It estimates the cost of using each index and picks the cheaper one

D. It uses both indexes simultaneously without cost estimation

Solution

1. Step 1: Understand index selection by cost-based optimizer

   The optimizer calculates the cost of using each index based on statistics like selectivity and size.

2. Step 2: Apply cost comparison to index choice

   It chooses the index that results in the lowest estimated cost for the query execution.

3. Final Answer:

   It estimates the cost of using each index and picks the cheaper one -> Option C

4. Quick Check:

   Index choice based on cost estimation [OK]

Hint: Optimizer picks index with lowest estimated cost [OK]

Common Mistakes:

• Assuming fixed index usage without cost check
• Thinking optimizer ignores indexes
• Believing it uses multiple indexes without cost analysis