EXPLAIN output reading in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Understanding the output of a simple EXPLAIN query

Given the following SQL query and its EXPLAIN output, what is the estimated number of rows the query planner expects to process?

EXPLAIN SELECT * FROM employees WHERE department_id = 5;

Output:

Seq Scan on employees  (cost=0.00..35.50 rows=10 width=100)

A 10

B 35

C 100

D 5

💡 Hint

Look for the 'rows=' value in the EXPLAIN output.

The 'rows=10' means the planner estimates 10 rows will be processed.

❓ query_result

intermediate

Identifying the join type from EXPLAIN output

Look at this EXPLAIN output for a join query. What type of join is being used?

Nested Loop  (cost=0.00..50.00 rows=20 width=200)
  -> Seq Scan on table1  (cost=0.00..20.00 rows=100 width=100)
  -> Index Scan using idx_table2 on table2  (cost=0.00..0.30 rows=1 width=100)

A Cross Join

B Hash Join

C Merge Join

D Nested Loop Join

💡 Hint

The first line of the EXPLAIN output usually shows the join type.

The output starts with 'Nested Loop', indicating a Nested Loop Join is used.

📝 Syntax

advanced

Detecting syntax error in EXPLAIN query usage

Which of the following EXPLAIN queries will cause a syntax error in PostgreSQL?

A EXPLAIN SELECT * FROM employees WHERE salary > 50000;

B EXPLAIN SELECT FROM employees WHERE salary > 50000;

C EXPLAIN (FORMAT JSON) SELECT * FROM employees WHERE salary > 50000;

D EXPLAIN ANALYZE SELECT * FROM employees WHERE salary > 50000;

💡 Hint

Check the syntax of the SELECT statement inside EXPLAIN.

Option B is missing the '*' after SELECT, causing a syntax error.

❓ optimization

advanced

Interpreting cost estimates in EXPLAIN output

Given this EXPLAIN output, what does the 'cost=0.00..100.00' represent?

Seq Scan on orders  (cost=0.00..100.00 rows=500 width=50)

A The startup cost is 0.00 and total cost is 100.00

B The query will take exactly 100 seconds to run

C The cost is the number of rows scanned

D The cost is the number of columns scanned

💡 Hint

Cost values show estimated resource usage, not time or row counts.

The first number is the startup cost, the second is the total estimated cost to run the query.

🧠 Conceptual

expert

Understanding the effect of EXPLAIN ANALYZE on query execution

What is the main difference between running EXPLAIN and EXPLAIN ANALYZE on a query in PostgreSQL?

A Both run the query but EXPLAIN ANALYZE shows more details about indexes

B EXPLAIN runs the query; EXPLAIN ANALYZE only shows the plan

C EXPLAIN shows estimated plan only; EXPLAIN ANALYZE runs the query and shows actual run times

D EXPLAIN ANALYZE disables all indexes during execution

💡 Hint

Think about whether the query is executed or just planned.

EXPLAIN only shows the estimated plan without running the query; EXPLAIN ANALYZE runs the query and shows actual execution details.

Practice

1. What does the EXPLAIN command in PostgreSQL primarily show?

easy

A. How PostgreSQL plans to execute a query

B. The exact data returned by the query

C. The syntax errors in the query

D. The database schema structure

Solution

1. Step 1: Understand the purpose of EXPLAIN

   EXPLAIN shows the query plan, which is how PostgreSQL intends to run the query.

2. Step 2: Differentiate from other commands

   It does not show actual data or errors, only the plan.

3. Final Answer:

   How PostgreSQL plans to execute a query -> Option A

4. Quick Check:

   EXPLAIN = query plan [OK]

Hint: EXPLAIN = query plan, not data or errors [OK]

Common Mistakes:

• Thinking EXPLAIN shows query results
• Confusing EXPLAIN with syntax error checks
• Assuming EXPLAIN shows database schema

2. Which of the following is the correct syntax to get the query plan for SELECT * FROM users; in PostgreSQL?

easy

A. EXPLAIN SELECT * FROM users;

B. EXPLAIN ANALYZE users SELECT *;

C. EXPLAIN FROM users SELECT *;

D. ANALYZE EXPLAIN SELECT * FROM users;

Solution

1. Step 1: Recall correct EXPLAIN syntax

   The correct syntax is EXPLAIN followed by the query.

2. Step 2: Check each option

   EXPLAIN SELECT * FROM users; matches the correct syntax. Others mix keywords incorrectly.

3. Final Answer:

   EXPLAIN SELECT * FROM users; -> Option A

4. Quick Check:

   EXPLAIN + query = correct syntax [OK]

Hint: EXPLAIN always precedes the query [OK]

Common Mistakes:

• Placing ANALYZE before EXPLAIN
• Using FROM before SELECT incorrectly
• Mixing keywords in wrong order

3. Given the EXPLAIN output below for SELECT * FROM orders WHERE customer_id = 5;, what does the line Index Scan using idx_customer_id on orders indicate?

medium

A. PostgreSQL is scanning the entire orders table

B. PostgreSQL is using an index to find matching rows

C. PostgreSQL is performing a sequential scan

D. PostgreSQL is creating a new index during query

Solution

1. Step 1: Understand 'Index Scan' meaning

   An Index Scan means PostgreSQL uses an index to quickly find rows matching the condition.

2. Step 2: Compare with other scan types

   Sequential scan means scanning all rows, which is not the case here.

3. Final Answer:

   PostgreSQL is using an index to find matching rows -> Option B

4. Quick Check:

   Index Scan = use index [OK]

Hint: 'Index Scan' means index used, not full table scan [OK]

Common Mistakes:

• Confusing Index Scan with Sequential Scan
• Thinking index is created during query
• Assuming full table scan always happens

4. You run EXPLAIN ANALYZE SELECT * FROM products WHERE price > 100; but get an error saying "relation 'products' does not exist." What is the likely cause?

medium

A. EXPLAIN ANALYZE cannot be used with WHERE clauses

B. The query syntax is incorrect for EXPLAIN ANALYZE

C. The table 'products' does not exist in the current database

D. You forgot to commit the transaction

Solution

1. Step 1: Analyze the error message

   The error says the table 'products' does not exist, meaning PostgreSQL cannot find it.

2. Step 2: Check other options

   EXPLAIN ANALYZE works with WHERE clauses and the syntax is correct. Committing transaction is unrelated.

3. Final Answer:

   The table 'products' does not exist in the current database -> Option C

4. Quick Check:

   Relation not found = missing table [OK]

Hint: Check table existence if 'relation does not exist' error appears [OK]

Common Mistakes:

• Assuming EXPLAIN ANALYZE disallows WHERE
• Blaming syntax when table is missing
• Thinking commit affects table visibility

5. You want to optimize a slow query. The EXPLAIN ANALYZE output shows a Seq Scan on a large table with a filter on a column. What is the best next step to improve performance?

hard

A. Drop the table and recreate it

B. Rewrite the query without the filter

C. Increase the work_mem setting

D. Create an index on the filtered column

Solution

1. Step 1: Understand Seq Scan impact

   A Seq Scan reads all rows, which is slow on large tables when filtering.

2. Step 2: Use index to speed filtering

   Creating an index on the filtered column lets PostgreSQL quickly find matching rows, avoiding full scan.

3. Final Answer:

   Create an index on the filtered column -> Option D

4. Quick Check:

   Seq Scan slow? Add index [OK]

Hint: Seq Scan slow? Add index on filter column [OK]

Common Mistakes:

• Removing filter instead of indexing
• Changing memory settings without indexing
• Dropping table unnecessarily