Query execution plans in DBMS Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the purpose of a query execution plan

What is the main purpose of a query execution plan in a database management system?

A To show the step-by-step operations the database will perform to execute a query

B To store the results of a query permanently

C To encrypt the data retrieved by a query

D To create a backup of the database before running a query

💡 Hint

Think about how the database decides to get data efficiently.

A query execution plan details the steps and methods the database engine uses to retrieve data for a query. It helps understand and optimize query performance.

📋 Factual

intermediate

Identifying components of a query execution plan

Which of the following is NOT typically part of a query execution plan?

A Data encryption algorithms

B Index scans

C Filter conditions

D Join methods

💡 Hint

Consider what a query execution plan focuses on: data retrieval, not security.

Query execution plans include index scans, join methods, and filter conditions to describe how data is accessed and combined. Encryption is unrelated to execution plans.

🔍 Analysis

advanced

Analyzing query execution plan output

Given a query execution plan showing a full table scan instead of an index scan, what is the most likely impact on query performance?

A There will be no difference in performance

B The query will run faster because full scans use less memory

C The query will likely run slower because scanning the whole table is less efficient

D The query will fail due to missing indexes

💡 Hint

Think about how much data the database reads in each case.

A full table scan reads every row, which is slower than using an index to find specific rows quickly. This usually causes slower query performance.

❓ Comparison

advanced

Comparing join methods in execution plans

Which join method in a query execution plan is generally more efficient for large tables with indexes available?

A Nested loop join

B Merge join

C Cartesian join

D Hash join

💡 Hint

Consider which join method benefits from sorted data and indexes.

Merge joins are efficient for large sorted datasets and can use indexes effectively. Nested loops are better for small datasets, hash joins for unsorted data, and Cartesian joins are usually inefficient.

❓ Reasoning

expert

Interpreting cost estimates in query execution plans

A query execution plan shows a high estimated cost for a particular operation. What does this cost represent?

A The number of rows returned by the query

B The monetary cost charged by the database provider for running the query

C The time in seconds the query will take to complete

D The estimated amount of resources like CPU and I/O the database expects to use for that operation

💡 Hint

Think about what 'cost' means in the context of query planning, not billing or exact time.

The cost in a query execution plan estimates the resources like CPU and disk I/O needed to perform an operation. It helps the optimizer choose the most efficient plan.