pg_stat_statements for slow queries in PostgreSQL

Introduction

pg_stat_statements helps you find slow queries in your PostgreSQL database. It shows which queries take the most time, so you can fix them and make your database faster.

You notice your application is slow and want to find which database queries cause delays.

You want to improve database performance by optimizing the slowest queries.

You want to monitor query performance regularly to catch problems early.

You are debugging a new feature and want to check if queries run efficiently.

You want to compare query speeds before and after changes to your database or code.

Syntax

PostgreSQL

SELECT query, calls, total_time, mean_time, max_time
FROM pg_stat_statements
ORDER BY total_time DESC
LIMIT 5;

This query shows the top 5 slowest queries by total execution time.

pg_stat_statements must be enabled in PostgreSQL to use this view.

Examples

Shows the top 3 queries with the highest total execution time.

PostgreSQL

SELECT query, calls, total_time
FROM pg_stat_statements
ORDER BY total_time DESC
LIMIT 3;

Shows the 5 queries with the highest average time per call, but only those run more than 10 times.

PostgreSQL

SELECT query, mean_time
FROM pg_stat_statements
WHERE calls > 10
ORDER BY mean_time DESC
LIMIT 5;

Finds the single query that took the longest time in any single execution.

PostgreSQL

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

Sample Program

This query lists the top 3 slowest queries by total time spent running. It helps you quickly see which queries use the most time overall.

PostgreSQL

SELECT query, calls, total_time, mean_time, max_time
FROM pg_stat_statements
ORDER BY total_time DESC
LIMIT 3;

Important Notes

You must enable the pg_stat_statements extension with CREATE EXTENSION pg_stat_statements; before using it.

Reset statistics with SELECT pg_stat_statements_reset(); if you want to clear old data.

pg_stat_statements tracks queries by normalizing them, so similar queries with different values are grouped together.

Summary

pg_stat_statements helps find slow queries by showing execution times and counts.

Use it to identify and fix queries that slow down your database.

Remember to enable the extension and reset stats when needed.

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