EXPLAIN plan for query analysis in SQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does this EXPLAIN output indicate about the query?

Given the EXPLAIN output below for a SELECT query, what does it tell you about how the database will execute the query?

EXPLAIN SELECT * FROM employees WHERE department_id = 5;

SQL

id | select_type | table     | type  | possible_keys | key         | key_len | ref   | rows | Extra
1  | SIMPLE      | employees | ref   | department_id | department_id | 4       | const | 10   | Using where

A The query performs a full join between employees and another table.

B The query will scan the entire employees table without using any index.

C The query uses a temporary table and filesort to order results.

D The query uses an index on department_id to find matching rows efficiently.

💡 Hint

Look at the 'key' column to see if an index is used.

The 'key' column shows 'department_id', meaning the query uses the index on department_id. The 'type' is 'ref', indicating an index lookup, not a full scan.

🧠 Conceptual

intermediate

What is the main purpose of using EXPLAIN in SQL?

Why do database developers use the EXPLAIN command before running complex queries?

A To see the query results before execution.

B To understand how the database will execute the query and optimize it.

C To create indexes automatically for the query.

D To delete duplicate rows from the result.

💡 Hint

Think about how EXPLAIN helps with performance.

EXPLAIN shows the query plan, helping developers see how tables are accessed and if indexes are used, so they can improve query speed.

📝 Syntax

advanced

Which EXPLAIN syntax is correct for analyzing a SELECT query in PostgreSQL?

Choose the correct syntax to get the execution plan for a SELECT query in PostgreSQL.

SQL

SELECT * FROM orders WHERE order_date > '2023-01-01';

A EXPLAIN PLAN FOR SELECT * FROM orders WHERE order_date > '2023-01-01';

B EXPLAIN QUERY SELECT * FROM orders WHERE order_date > '2023-01-01';

C EXPLAIN SELECT * FROM orders WHERE order_date > '2023-01-01';

D EXPLAIN EXECUTE SELECT * FROM orders WHERE order_date > '2023-01-01';

💡 Hint

PostgreSQL uses a simple EXPLAIN keyword before the query.

In PostgreSQL, the correct syntax is 'EXPLAIN' followed by the query. Other options are invalid or from other database systems.

❓ optimization

advanced

How can EXPLAIN help improve a slow query with multiple joins?

You have a slow query joining three large tables. How does EXPLAIN help you improve it?

A It shows which join order and indexes the database uses, helping you adjust indexes or rewrite the query.

B It automatically rewrites the query to be faster.

C It deletes unnecessary rows from the tables to speed up the query.

D It caches the query results for faster future runs.

💡 Hint

EXPLAIN shows the plan but does not change the query.

EXPLAIN reveals the join order, index usage, and scan types, so you can add indexes or rewrite joins to improve speed.

🔧 Debug

expert

What error or issue does this EXPLAIN output reveal about the query?

EXPLAIN output for a query shows:

id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra 1 | SIMPLE | orders | ALL | NULL | NULL | NULL | NULL | 10000 | Using where

What does this indicate?

A The query uses a full table scan on orders, which may cause slow performance.

B The query uses an index to quickly find matching rows.

C The query is using a temporary table and filesort.

D The query has a syntax error and will not run.

💡 Hint

Look at the 'type' and 'key' columns.

The 'type' is 'ALL' and 'key' is NULL, meaning no index is used and the entire table is scanned, which can be slow for large tables.

Practice

1. What is the main purpose of using EXPLAIN before running a SQL query?

easy

A. To see how the database plans to execute the query

B. To delete data from the database

C. To create a new table

D. To insert new rows into a table

Solution

1. Step 1: Understand the role of EXPLAIN

   EXPLAIN shows the steps the database will take to run a query, like a map of the plan.

2. Step 2: Identify the correct purpose

   It helps find slow parts and optimize queries, not to modify data or structure.

3. Final Answer:

   To see how the database plans to execute the query -> Option A

4. Quick Check:

   EXPLAIN = query plan [OK]

Hint: EXPLAIN shows query steps, not data changes [OK]

Common Mistakes:

• Thinking EXPLAIN modifies data
• Confusing EXPLAIN with data manipulation commands
• Assuming EXPLAIN creates or deletes tables

2. Which of the following is the correct syntax to get the execution plan of a query in SQL?

easy

A. EXPLAIN SELECT * FROM users;

B. PLAN EXPLAIN SELECT * FROM users;

C. SHOW PLAN FOR SELECT * FROM users;

D. RUN EXPLAIN ON SELECT * FROM users;

Solution

1. Step 1: Recall standard EXPLAIN syntax

   The standard way to get a query plan is to put EXPLAIN before the query.

2. Step 2: Check each option

   Only EXPLAIN SELECT * FROM users; uses the correct syntax: EXPLAIN SELECT ...; others are invalid or non-standard.

3. Final Answer:

   EXPLAIN SELECT * FROM users; -> Option A

4. Quick Check:

   EXPLAIN + query = correct syntax [OK]

Hint: Put EXPLAIN before the query to see the plan [OK]

Common Mistakes:

• Adding extra keywords like PLAN or RUN
• Using SHOW PLAN which is not standard SQL
• Placing EXPLAIN after the query

3. Given the query SELECT * FROM orders WHERE customer_id = 5;, what does the EXPLAIN plan typically show?

medium

A. The syntax errors in the query

B. The total number of rows in the orders table

C. The list of all customers in the database

D. The steps to scan the orders table and filter rows where customer_id = 5

Solution

1. Step 1: Understand what EXPLAIN outputs

   EXPLAIN shows how the database will find rows, like scanning and filtering steps.

2. Step 2: Match the query with the plan description

   The query filters orders by customer_id = 5, so the plan shows scanning and filtering those rows.

3. Final Answer:

   The steps to scan the orders table and filter rows where customer_id = 5 -> Option D

4. Quick Check:

   EXPLAIN = scan + filter steps [OK]

Hint: EXPLAIN shows how rows are found and filtered [OK]

Common Mistakes:

• Thinking EXPLAIN shows data counts
• Expecting EXPLAIN to list unrelated tables
• Confusing EXPLAIN output with errors

4. You run EXPLAIN SELECT name FROM employees WHERE age > 30; but get an error. What is the most likely cause?

medium

A. The EXPLAIN keyword is not supported in your SQL dialect

B. The query is missing a semicolon at the end

C. The table employees does not exist

D. The WHERE clause uses an invalid operator

Solution

1. Step 1: Check common EXPLAIN errors

   EXPLAIN syntax is usually supported; missing semicolon rarely causes errors in many tools.

2. Step 2: Consider table existence

   If the table does not exist, EXPLAIN cannot analyze the query and will error.

3. Final Answer:

   The table employees does not exist -> Option C

4. Quick Check:

   Missing table = EXPLAIN error [OK]

Hint: Check if tables exist before EXPLAIN [OK]

Common Mistakes:

• Assuming EXPLAIN is unsupported
• Blaming missing semicolon
• Thinking WHERE operator is invalid

5. You want to optimize a slow query joining two large tables. How can using EXPLAIN help you improve performance?

hard

A. By deleting duplicate rows from the tables

B. By showing which table is scanned first and if indexes are used

C. By automatically rewriting the query for better speed

D. By increasing the database server memory

Solution

1. Step 1: Understand EXPLAIN's role in optimization

   EXPLAIN shows the order of table scans and whether indexes help speed up lookups.

2. Step 2: Identify how this helps optimization

   Knowing scan order and index use helps rewrite queries or add indexes to improve speed.

3. Final Answer:

   By showing which table is scanned first and if indexes are used -> Option B

4. Quick Check:

   EXPLAIN reveals scan order and index use [OK]

Hint: EXPLAIN reveals scan order and index usage [OK]

Common Mistakes:

• Expecting EXPLAIN to fix queries automatically
• Confusing EXPLAIN with data cleanup
• Thinking EXPLAIN changes server settings