EXPLAIN ANALYZE for actual execution in PostgreSQL - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to show the actual execution plan of a query.

PostgreSQL

EXPLAIN [1] SELECT * FROM employees;

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

AANALYZE

BPLAN

CQUERY

DEXECUTE

Attempts:

3 left

💡 Hint

Common Mistakes

Using EXPLAIN PLAN instead of EXPLAIN ANALYZE.

Using EXPLAIN EXECUTE which is not valid.

The EXPLAIN ANALYZE command runs the query and shows the actual execution plan with timing.

2fill in blank

medium

Complete the code to get the execution time of a SELECT query on the products table.

PostgreSQL

EXPLAIN ANALYZE SELECT [1] FROM products WHERE price > 100;

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

Aid

Bprice

C*

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Selecting only one column limits the plan details.

Using column names instead of * when full output is needed.

Using * selects all columns, so the plan shows the full query execution.

3fill in blank

hard

Fix the error in the EXPLAIN ANALYZE command to run it correctly.

PostgreSQL

EXPLAIN ANALYZE [1] FROM orders;

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

ASELECT *

BDELETE

CUPDATE

DINSERT

Attempts:

3 left

💡 Hint

Common Mistakes

Using only FROM without SELECT causes syntax error.

Using DELETE or UPDATE without full statement.

The EXPLAIN ANALYZE command must be followed by a valid SELECT, INSERT, UPDATE, or DELETE statement. Here, SELECT * is correct to query the table.

4fill in blank

hard

Fill both blanks to explain and analyze a query that counts rows in the sales table.

PostgreSQL

EXPLAIN [1] SELECT COUNT([2]) FROM sales;

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

AANALYZE

Bprice

C*

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using a column name inside COUNT instead of * counts only non-null values.

Omitting ANALYZE shows only estimated plan, not actual execution.

Use ANALYZE to run the query and get actual execution details. Use * inside COUNT to count all rows.

5fill in blank

hard

Fill all three blanks to explain and analyze a query that selects names and filters by age in the users table.

PostgreSQL

EXPLAIN [1] SELECT name FROM users WHERE age [2] [3];

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

AANALYZE

B>

C30

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of > changes the filter logic.

Omitting ANALYZE shows only estimated plan.

Use ANALYZE to run the query and get real execution info. The condition age > 30 filters users older than 30.

Practice

1. What is the main purpose of using EXPLAIN ANALYZE in PostgreSQL?

easy

A. To only check the syntax of the SQL query without running it

B. To run the query and show the actual execution plan with timing details

C. To delete data from the database safely

D. To create a backup of the database

Solution

1. Step 1: Understand what EXPLAIN ANALYZE does

   It runs the SQL query and collects detailed information about how the database executes it, including timing and row counts.

2. Step 2: Compare with other options

   Options A, C, and D describe different actions unrelated to EXPLAIN ANALYZE's purpose.

3. Final Answer:

   To run the query and show the actual execution plan with timing details -> Option B

4. Quick Check:

   EXPLAIN ANALYZE = actual execution plan with timing [OK]

Hint: EXPLAIN ANALYZE runs query and shows real execution details [OK]

Common Mistakes:

• Thinking EXPLAIN ANALYZE only checks syntax
• Confusing it with backup or delete commands
• Assuming it does not run the query

2. Which of the following is the correct syntax to get the actual execution plan of a query SELECT * FROM users; using EXPLAIN ANALYZE?

easy

A. EXPLAIN SELECT * FROM users;

B. EXPLAIN RUN SELECT * FROM users;

C. EXPLAIN ANALYZE SELECT * FROM users;

D. ANALYZE EXPLAIN SELECT * FROM users;

Solution

1. Step 1: Recall the correct EXPLAIN ANALYZE syntax

   The correct syntax is to write EXPLAIN ANALYZE followed by the query to run and analyze.

2. Step 2: Check each option

   EXPLAIN SELECT * FROM users; misses ANALYZE, so it only shows estimated plan. ANALYZE EXPLAIN SELECT * FROM users; reverses keywords incorrectly. EXPLAIN RUN SELECT * FROM users; uses an invalid keyword RUN.

3. Final Answer:

   EXPLAIN ANALYZE SELECT * FROM users; -> Option C

4. Quick Check:

   EXPLAIN ANALYZE + query = correct syntax [OK]

Hint: EXPLAIN ANALYZE before query runs it and shows plan [OK]

Common Mistakes:

• Omitting ANALYZE keyword
• Swapping EXPLAIN and ANALYZE order
• Using invalid keywords like RUN

3. Given the query EXPLAIN ANALYZE SELECT * FROM orders WHERE order_id = 10;, which part of the output tells you how many rows were actually returned?

medium

A. The "Buffers" section

B. The "Estimated Rows" value in the output

C. The "Planning Time" value

D. The "Actual Rows" value in the output

Solution

1. Step 1: Understand EXPLAIN ANALYZE output fields

   "Actual Rows" shows the real number of rows returned by each step during execution.

2. Step 2: Differentiate from other fields

   "Estimated Rows" is the planner's guess before running. "Planning Time" is time spent planning. "Buffers" shows disk usage, not row count.

3. Final Answer:

   The "Actual Rows" value in the output -> Option D

4. Quick Check:

   Actual Rows = real returned rows [OK]

Hint: "Actual Rows" shows real returned rows, not estimates [OK]

Common Mistakes:

• Confusing estimated rows with actual rows
• Looking at planning time for row count
• Ignoring actual execution details

4. You run ANALYZE EXPLAIN SELECT * FROM products WHERE price > 100; but get an error: ERROR: syntax error at or near "EXPLAIN". What is the likely cause?

medium

A. You wrote ANALYZE before EXPLAIN

B. You forgot the semicolon at the end

C. You used EXPLAIN without ANALYZE

D. You ran the query without EXPLAIN

Solution

1. Step 1: Analyze the error message

   The error points to a syntax problem near "EXPLAIN" which suggests wrong keyword order.

2. Step 2: Check correct keyword order

   The correct order is EXPLAIN ANALYZE, not ANALYZE EXPLAIN. Writing ANALYZE first causes syntax error.

3. Final Answer:

   You wrote ANALYZE before EXPLAIN -> Option A

4. Quick Check:

   EXPLAIN must come before ANALYZE [OK]

Hint: EXPLAIN always comes before ANALYZE in syntax [OK]

Common Mistakes:

• Swapping EXPLAIN and ANALYZE keywords
• Missing semicolon (usually different error)
• Running query without EXPLAIN ANALYZE

5. You want to optimize a slow query joining customers and orders. Using EXPLAIN ANALYZE, you see a sequential scan on orders despite an index on customer_id. What should you check or do next?

hard

A. Check if statistics are outdated and run ANALYZE orders;

B. Drop the index on customer_id to force sequential scan

C. Rewrite the query without JOIN to avoid scans

D. Increase the database cache size to ignore indexes

Solution

1. Step 1: Understand why index might be ignored

   If statistics are outdated, the planner may wrongly choose a sequential scan instead of using the index.

2. Step 2: Use ANALYZE to update statistics

   Running ANALYZE orders; updates table statistics so planner can make better decisions.

3. Final Answer:

   Check if statistics are outdated and run ANALYZE orders; -> Option A

4. Quick Check:

   Update stats with ANALYZE to help planner use indexes [OK]

Hint: Run ANALYZE to update stats so planner uses indexes [OK]

Common Mistakes:

• Dropping useful indexes
• Ignoring statistics update
• Trying to avoid JOINs instead of fixing plan