pg_stat_statements for slow queries in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select the query text from pg_stat_statements.

PostgreSQL

SELECT [1] FROM pg_stat_statements;

Drag options to blanks, or click blank then click option'

Aquery

Buserid

Ccalls

Dtotal_time

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting 'userid' instead of 'query' will not show the SQL text.

Selecting 'calls' or 'total_time' shows statistics, not the query text.

The query column contains the text of the SQL statements tracked by pg_stat_statements.

2fill in blank

medium

Complete the code to order queries by total execution time descending.

PostgreSQL

SELECT query, total_time FROM pg_stat_statements ORDER BY [1] DESC;

Drag options to blanks, or click blank then click option'

Acalls

Bmean_time

Cuserid

Dtotal_time

Attempts:

3 left

💡 Hint

Common Mistakes

Ordering by 'calls' orders by number of executions, not time.

Ordering by 'mean_time' orders by average time, not total.

Ordering by total_time descending shows the slowest queries first by total time spent.

3fill in blank

hard

Fix the error in the code to filter queries with total_time greater than 1000 ms.

PostgreSQL

SELECT query, total_time FROM pg_stat_statements WHERE total_time [1] 1000;

Drag options to blanks, or click blank then click option'

A>

B=

C<

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '=' will only find queries exactly 1000 ms, which is unlikely.

Using '<' or '<=' finds faster queries, not slow ones.

To find slow queries, filter where total_time is greater than 1000 milliseconds.

4fill in blank

hard

Fill both blanks to select query and calls where calls are more than 50 and order by calls descending.

PostgreSQL

SELECT [1], calls FROM pg_stat_statements WHERE calls [2] 50 ORDER BY calls DESC;

Drag options to blanks, or click blank then click option'

Aquery

B>

C<

Dtotal_time

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting 'total_time' instead of 'query' will not show the SQL text.

Using '<' instead of '>' filters for fewer calls, not more.

Select query and filter where calls is greater than 50 to find frequently run queries, then order by calls descending.

5fill in blank

hard

Fill all three blanks to select query, calls, and mean_time for queries with mean_time over 200 ms, ordered by mean_time descending.

PostgreSQL

SELECT [1], [2], mean_time FROM pg_stat_statements WHERE mean_time [3] 200 ORDER BY mean_time DESC;

Drag options to blanks, or click blank then click option'

Aquery

Bcalls

C>

Dtotal_time

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting 'total_time' instead of 'calls' changes the meaning.

Using '<' instead of '>' filters for faster queries, not slower.

Select query and calls, filter where mean_time is greater than 200 ms, and order by mean_time descending to find slow queries by average time.

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