pg_stat_statements for slow queries in PostgreSQL - Mini Project: Build & Apply

Using pg_stat_statements to Identify Slow Queries in PostgreSQL

📖 Scenario: You are a database administrator for a small online store. You want to find out which queries are running slowly so you can improve the website's speed.

🎯 Goal: Learn how to enable and query the pg_stat_statements extension in PostgreSQL to find slow queries.

📋 What You'll Learn

Enable the pg_stat_statements extension

Create a configuration variable for minimum execution time

Write a query to select slow queries from pg_stat_statements

Order the results by average execution time descending

💡 Why This Matters

🌍 Real World

Database administrators use pg_stat_statements to monitor and improve query performance in real applications.

💼 Career

Knowing how to find slow queries helps optimize databases, a key skill for database administrators and backend developers.

1

Enable the pg_stat_statements extension

Write the SQL command to enable the pg_stat_statements extension in your current database.

PostgreSQL

-- Your code here

Hint

Use CREATE EXTENSION IF NOT EXISTS pg_stat_statements; to enable the extension.

2

Set minimum execution time for slow queries

Create a variable called min_exec_time and set it to 100 to represent 100 milliseconds as the threshold for slow queries.

PostgreSQL

CREATE EXTENSION IF NOT EXISTS pg_stat_statements;
-- Your code here

Hint

Use \set min_exec_time 100 to create the variable in psql.

3

Query slow queries from pg_stat_statements

Write a SQL query to select query, calls, and mean_exec_time from pg_stat_statements where mean_exec_time is greater than :min_exec_time.

PostgreSQL

CREATE EXTENSION IF NOT EXISTS pg_stat_statements;
\set min_exec_time 100
-- Your code here

Hint

Use SELECT query, calls, mean_exec_time FROM pg_stat_statements WHERE mean_exec_time > :min_exec_time;

4

Order slow queries by average execution time

Modify the previous query to order the results by mean_exec_time in descending order.

PostgreSQL

CREATE EXTENSION IF NOT EXISTS pg_stat_statements;
\set min_exec_time 100
SELECT query, calls, mean_exec_time
FROM pg_stat_statements
WHERE mean_exec_time > :min_exec_time
-- Your code here

Hint

Add ORDER BY mean_exec_time DESC to sort slow queries from slowest to fastest.

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