EXPLAIN plan for query analysis in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the EXPLAIN plan in SQL?

The EXPLAIN plan shows how the database will execute a query. It helps understand the steps and order the database uses to get the results.

beginner

Which part of the EXPLAIN plan tells you the order of operations?

The EXPLAIN plan lists operations in the order the database will perform them, usually from bottom to top or left to right, showing the flow of data.

beginner

What does a 'Seq Scan' mean in an EXPLAIN plan?

A 'Seq Scan' means the database reads the whole table row by row to find matching data. It can be slow for big tables.

intermediate

How can EXPLAIN help improve query performance?

By showing how a query runs, EXPLAIN helps find slow parts like full table scans. You can then add indexes or rewrite queries to make them faster.

intermediate

What does the 'cost' in an EXPLAIN plan represent?

The 'cost' is a number estimating how much work the database needs to do for each step. Lower cost usually means faster execution.

What does the EXPLAIN plan show for a SQL query?

A The exact time the query took to run

B The syntax errors in the query

C The number of rows returned by the query

D How the database will execute the query

EXPLAIN shows the execution steps, not runtime or errors.

In an EXPLAIN plan, what does a 'Seq Scan' indicate?

A Sorting the results

B Using an index to find rows quickly

C A full table scan reading all rows

D Joining two tables

'Seq Scan' means scanning the whole table row by row.

Why is the 'cost' value important in an EXPLAIN plan?

A It measures the query's memory usage

B It estimates the work needed to run the query step

C It counts the rows returned

D It shows the number of errors in the query

'Cost' estimates how much work the database does for each step.

How can EXPLAIN help you improve a slow query?

A By showing which parts take the most work

B By fixing syntax errors automatically

C By increasing the database size

D By deleting unnecessary tables

EXPLAIN helps identify slow parts to optimize.

Which of these is NOT shown by an EXPLAIN plan?

A Actual query execution time

B Estimated cost

C Order of operations

D Type of scans used

EXPLAIN shows estimated plans, not actual run times.

Explain what an EXPLAIN plan is and how it helps with SQL queries.

Think about how you would check the steps a recipe follows before cooking.

Describe what a 'Seq Scan' means in an EXPLAIN plan and why it might be slow.

Imagine reading every page of a book to find one word.

Practice

1. What is the main purpose of using EXPLAIN before running a SQL query?

easy

A. To see how the database plans to execute the query

B. To delete data from the database

C. To create a new table

D. To insert new rows into a table

Solution

1. Step 1: Understand the role of EXPLAIN

   EXPLAIN shows the steps the database will take to run a query, like a map of the plan.

2. Step 2: Identify the correct purpose

   It helps find slow parts and optimize queries, not to modify data or structure.

3. Final Answer:

   To see how the database plans to execute the query -> Option A

4. Quick Check:

   EXPLAIN = query plan [OK]

Hint: EXPLAIN shows query steps, not data changes [OK]

Common Mistakes:

• Thinking EXPLAIN modifies data
• Confusing EXPLAIN with data manipulation commands
• Assuming EXPLAIN creates or deletes tables

2. Which of the following is the correct syntax to get the execution plan of a query in SQL?

easy

A. EXPLAIN SELECT * FROM users;

B. PLAN EXPLAIN SELECT * FROM users;

C. SHOW PLAN FOR SELECT * FROM users;

D. RUN EXPLAIN ON SELECT * FROM users;

Solution

1. Step 1: Recall standard EXPLAIN syntax

   The standard way to get a query plan is to put EXPLAIN before the query.

2. Step 2: Check each option

   Only EXPLAIN SELECT * FROM users; uses the correct syntax: EXPLAIN SELECT ...; others are invalid or non-standard.

3. Final Answer:

   EXPLAIN SELECT * FROM users; -> Option A

4. Quick Check:

   EXPLAIN + query = correct syntax [OK]

Hint: Put EXPLAIN before the query to see the plan [OK]

Common Mistakes:

• Adding extra keywords like PLAN or RUN
• Using SHOW PLAN which is not standard SQL
• Placing EXPLAIN after the query

3. Given the query SELECT * FROM orders WHERE customer_id = 5;, what does the EXPLAIN plan typically show?

medium

A. The syntax errors in the query

B. The total number of rows in the orders table

C. The list of all customers in the database

D. The steps to scan the orders table and filter rows where customer_id = 5

Solution

1. Step 1: Understand what EXPLAIN outputs

   EXPLAIN shows how the database will find rows, like scanning and filtering steps.

2. Step 2: Match the query with the plan description

   The query filters orders by customer_id = 5, so the plan shows scanning and filtering those rows.

3. Final Answer:

   The steps to scan the orders table and filter rows where customer_id = 5 -> Option D

4. Quick Check:

   EXPLAIN = scan + filter steps [OK]

Hint: EXPLAIN shows how rows are found and filtered [OK]

Common Mistakes:

• Thinking EXPLAIN shows data counts
• Expecting EXPLAIN to list unrelated tables
• Confusing EXPLAIN output with errors

4. You run EXPLAIN SELECT name FROM employees WHERE age > 30; but get an error. What is the most likely cause?

medium

A. The EXPLAIN keyword is not supported in your SQL dialect

B. The query is missing a semicolon at the end

C. The table employees does not exist

D. The WHERE clause uses an invalid operator

Solution

1. Step 1: Check common EXPLAIN errors

   EXPLAIN syntax is usually supported; missing semicolon rarely causes errors in many tools.

2. Step 2: Consider table existence

   If the table does not exist, EXPLAIN cannot analyze the query and will error.

3. Final Answer:

   The table employees does not exist -> Option C

4. Quick Check:

   Missing table = EXPLAIN error [OK]

Hint: Check if tables exist before EXPLAIN [OK]

Common Mistakes:

• Assuming EXPLAIN is unsupported
• Blaming missing semicolon
• Thinking WHERE operator is invalid

5. You want to optimize a slow query joining two large tables. How can using EXPLAIN help you improve performance?

hard

A. By deleting duplicate rows from the tables

B. By showing which table is scanned first and if indexes are used

C. By automatically rewriting the query for better speed

D. By increasing the database server memory

Solution

1. Step 1: Understand EXPLAIN's role in optimization

   EXPLAIN shows the order of table scans and whether indexes help speed up lookups.

2. Step 2: Identify how this helps optimization

   Knowing scan order and index use helps rewrite queries or add indexes to improve speed.

3. Final Answer:

   By showing which table is scanned first and if indexes are used -> Option B

4. Quick Check:

   EXPLAIN reveals scan order and index use [OK]

Hint: EXPLAIN reveals scan order and index usage [OK]

Common Mistakes:

• Expecting EXPLAIN to fix queries automatically
• Confusing EXPLAIN with data cleanup
• Thinking EXPLAIN changes server settings