pg_stat_statements for slow queries in PostgreSQL - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Identify the slowest query using pg_stat_statements

Given the pg_stat_statements view, which query will return the SQL statement with the highest average execution time?

PostgreSQL

SELECT query, mean_time FROM pg_stat_statements ORDER BY mean_time DESC LIMIT 1;

A SELECT query, calls FROM pg_stat_statements ORDER BY calls DESC LIMIT 1;

B SELECT query, total_time FROM pg_stat_statements ORDER BY total_time ASC LIMIT 1;

C SELECT query, mean_time FROM pg_stat_statements ORDER BY mean_time DESC LIMIT 1;

D SELECT query, mean_time FROM pg_stat_statements ORDER BY mean_time ASC LIMIT 1;

💡 Hint

Think about which column shows the average time per execution and how to get the highest value.

The mean_time column shows the average execution time of each query. Ordering by it descending and limiting to 1 returns the slowest average query.

🧠 Conceptual

intermediate

Understanding pg_stat_statements columns

Which column in pg_stat_statements helps identify how many times a query was executed?

A calls

B mean_time

C rows

D total_time

💡 Hint

Look for the column that counts executions.

The calls column shows how many times the query was executed.

📝 Syntax

advanced

Correct syntax to reset pg_stat_statements statistics

Which SQL command correctly resets all statistics collected by pg_stat_statements?

A SELECT pg_stat_statements_reset();

B RESET pg_stat_statements;

C TRUNCATE pg_stat_statements;

D DELETE FROM pg_stat_statements;

💡 Hint

Resetting statistics is done by a function call, not by deleting or truncating.

The function pg_stat_statements_reset() resets all statistics collected by the extension.

❓ optimization

advanced

Optimizing query to find slow queries with high calls

You want to find queries that are both slow on average and executed many times. Which query correctly selects queries with average time over 100ms and calls over 1000?

PostgreSQL

SELECT query, mean_time, calls FROM pg_stat_statements WHERE mean_time > 100 AND calls > 1000 ORDER BY mean_time DESC;

A SELECT query, mean_time, calls FROM pg_stat_statements WHERE mean_time > 100 AND calls < 1000 ORDER BY calls DESC;

B SELECT query, mean_time, calls FROM pg_stat_statements WHERE mean_time > 100 AND calls > 1000 ORDER BY mean_time DESC;

C SELECT query, mean_time, calls FROM pg_stat_statements WHERE mean_time < 100 AND calls > 1000 ORDER BY mean_time DESC;

D SELECT query, mean_time, calls FROM pg_stat_statements WHERE mean_time >= 100 OR calls >= 1000 ORDER BY calls DESC;

💡 Hint

Use AND to combine conditions that must both be true.

Option B correctly filters queries with mean_time greater than 100ms and calls greater than 1000, ordering by slowest first.

🔧 Debug

expert

Diagnose error when querying pg_stat_statements

You run this query but get an error: ERROR: relation "pg_stat_statements" does not exist. What is the most likely cause?

A You need to prefix pg_stat_statements with the schema name 'public'.

B The query syntax is incorrect; pg_stat_statements is a function, not a table.

C The user does not have SELECT permission on pg_stat_statements.

D The pg_stat_statements extension is not installed or enabled in the current database.

💡 Hint

Check if the extension is installed and enabled.

The error usually means the extension is not installed or enabled in the current database. You must run CREATE EXTENSION pg_stat_statements; first.

Practice

1. What is the primary purpose of the pg_stat_statements extension in PostgreSQL?

easy

A. To manage user permissions and roles

B. To backup the database automatically

C. To optimize disk storage usage

D. To track and report query execution statistics including slow queries

Solution

1. Step 1: Understand the role of pg_stat_statements

   The extension collects statistics about query execution times and counts, helping identify slow queries.

2. Step 2: Compare with other options

   Options A, B, and D describe unrelated database functions like backup, permissions, and storage optimization.

3. Final Answer:

   To track and report query execution statistics including slow queries -> Option D

4. Quick Check:

   pg_stat_statements = track slow queries [OK]

Hint: Remember: pg_stat_statements tracks query performance stats [OK]

Common Mistakes:

• Confusing pg_stat_statements with backup tools
• Thinking it manages user roles
• Assuming it optimizes disk space

2. Which SQL command correctly enables the pg_stat_statements extension in PostgreSQL?

easy

A. CREATE EXTENSION pg_stat_statements;

B. ENABLE EXTENSION pg_stat_statements;

C. LOAD EXTENSION pg_stat_statements;

D. START EXTENSION pg_stat_statements;

Solution

1. Step 1: Recall the syntax to enable extensions

   PostgreSQL uses CREATE EXTENSION extension_name; to enable extensions.

2. Step 2: Check other options

   Commands like ENABLE, LOAD, or START are not valid for enabling extensions in PostgreSQL.

3. Final Answer:

   CREATE EXTENSION pg_stat_statements; -> Option A

4. Quick Check:

   Enable extension = CREATE EXTENSION [OK]

Hint: Use CREATE EXTENSION to enable PostgreSQL extensions [OK]

Common Mistakes:

• Using ENABLE or LOAD instead of CREATE EXTENSION
• Forgetting the semicolon at the end
• Trying to enable without superuser rights

3. Given the query:

SELECT query, total_time, calls FROM pg_stat_statements ORDER BY total_time DESC LIMIT 1;

What does this query return?

medium

A. The query with the fewest calls

B. The most recent query executed

C. The query with the highest total execution time and its stats

D. All queries sorted by average time

Solution

1. Step 1: Analyze the ORDER BY clause

   The query orders results by total_time in descending order, so the highest total execution time is first.

2. Step 2: Understand the LIMIT 1

   LIMIT 1 returns only the top row, which is the slowest query by total execution time.

3. Final Answer:

   The query with the highest total execution time and its stats -> Option C

4. Quick Check:

   ORDER BY total_time DESC LIMIT 1 = slowest query [OK]

Hint: ORDER BY total_time DESC LIMIT 1 shows slowest query [OK]

Common Mistakes:

• Thinking it returns the most recent query
• Confusing total_time with average time
• Assuming it returns all queries

4. You run this query:

SELECT * FROM pg_stat_statements WHERE query = 'SELECT * FROM users';

But it returns no rows. What could be the problem?

medium

A. The exact query text does not match due to whitespace or formatting differences

B. pg_stat_statements is disabled and not collecting data

C. The users table does not exist

D. The query is too fast to be recorded

Solution

1. Step 1: Understand how pg_stat_statements stores queries

   It stores normalized query texts, so exact string matches may fail if whitespace or formatting differs.

2. Step 2: Consider other options

   While B is possible, the question implies pg_stat_statements is enabled. C and D do not explain no rows for that query text.

3. Final Answer:

   The exact query text does not match due to whitespace or formatting differences -> Option A

4. Quick Check:

   Exact query text match may fail due to formatting [OK]

Hint: Query text must match exactly including spaces [OK]

Common Mistakes:

• Assuming any query text matches regardless of formatting
• Ignoring that extension might be disabled
• Thinking table existence affects pg_stat_statements output

5. You want to reset all collected statistics in pg_stat_statements to start fresh after fixing slow queries. Which command should you run?

hard

A. TRUNCATE pg_stat_statements;

B. SELECT pg_stat_statements_reset();

C. DROP EXTENSION pg_stat_statements;

D. RESET pg_stat_statements;

Solution

1. Step 1: Identify the correct function to reset stats

   PostgreSQL provides the function pg_stat_statements_reset() to clear collected statistics.

2. Step 2: Evaluate other options

   RESET is not valid syntax here, DROP EXTENSION removes the extension, and TRUNCATE is not allowed on this view.

3. Final Answer:

   SELECT pg_stat_statements_reset(); -> Option B

4. Quick Check:

   Reset stats = pg_stat_statements_reset() function [OK]

Hint: Use SELECT pg_stat_statements_reset() to clear stats [OK]

Common Mistakes:

• Trying to DROP the extension to reset stats
• Using RESET command incorrectly
• Attempting to TRUNCATE the stats view