EXPLAIN query analysis in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the EXPLAIN statement do in MySQL?

EXPLAIN shows how MySQL executes a query. It helps understand the query plan and performance.

intermediate

What is the meaning of the 'type' column in EXPLAIN output?

The 'type' column shows the join type or how MySQL finds rows. Types range from 'ALL' (slow) to 'const' (fast).

beginner

What does the 'rows' column in EXPLAIN output represent?

It estimates how many rows MySQL will examine to execute the query part.

beginner

Why is it important to check the 'key' column in EXPLAIN output?

The 'key' column shows which index MySQL uses. Using indexes speeds up queries.

intermediate

What does it mean if EXPLAIN shows 'Using temporary' in the 'Extra' column?

It means MySQL creates a temporary table to process the query, which can slow down performance.

What is the main purpose of the EXPLAIN statement in MySQL?

A To show how MySQL executes a query

B To delete data from a table

C To create a new database

D To backup the database

EXPLAIN helps understand the query execution plan, not data modification or database management.

In EXPLAIN output, which 'type' value indicates the slowest way to read data?

A const

B ref

C ALL

D eq_ref

'ALL' means a full table scan, which is the slowest method.

What does the 'key' column in EXPLAIN output tell you?

A The query execution time

B The number of rows returned

C The table name

D The index MySQL uses for the query

'key' shows which index is used to speed up the query.

If EXPLAIN shows 'Using temporary' in the Extra column, what does it mean?

A The query is invalid

B MySQL is using a temporary table

C No indexes are used

D The query is very fast

'Using temporary' means MySQL creates a temporary table to process the query.

Which EXPLAIN column estimates how many rows MySQL will check?

A rows

B select_type

C id

D Extra

The 'rows' column estimates the number of rows MySQL will examine.

Explain what information the EXPLAIN statement provides about a MySQL query.

Think about how EXPLAIN helps you understand query performance.

Describe why checking the EXPLAIN output is useful before optimizing a query.

Consider what problems EXPLAIN can help you find.

Practice

1. What is the main purpose of using EXPLAIN before a SELECT query in MySQL?

easy

A. To create a backup of the database

B. To run the query faster by optimizing it automatically

C. To delete rows from the table before selecting

D. To see how MySQL plans to execute the query and find performance issues

Solution

1. Step 1: Understand the role of EXPLAIN

   EXPLAIN shows the execution plan MySQL uses for a query, helping to understand how tables and indexes are accessed.

2. Step 2: Identify what EXPLAIN does not do

   EXPLAIN does not run the query faster, delete data, or create backups; it only provides information about query execution.

3. Final Answer:

   To see how MySQL plans to execute the query and find performance issues -> Option D

4. Quick Check:

   EXPLAIN shows query plan = D [OK]

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

Common Mistakes:

• Thinking EXPLAIN runs the query faster
• Confusing EXPLAIN with data modification commands
• Assuming EXPLAIN creates backups

2. Which of the following is the correct syntax to use EXPLAIN with a SELECT query in MySQL?

easy

A. EXPLAIN * SELECT FROM users;

B. EXPLAIN SELECT * FROM users;

C. SELECT EXPLAIN * FROM users;

D. SELECT * FROM users EXPLAIN;

Solution

1. Step 1: Recall the correct EXPLAIN syntax

   The correct syntax is to write EXPLAIN before the SELECT statement: EXPLAIN SELECT ....

2. Step 2: Check each option

   EXPLAIN SELECT * FROM users; uses the correct syntax. Options A, B, and D place EXPLAIN incorrectly and will cause syntax errors.

3. Final Answer:

   EXPLAIN SELECT * FROM users; -> Option B

4. Quick Check:

   EXPLAIN before SELECT = C [OK]

Hint: Put EXPLAIN right before SELECT to check query plan [OK]

Common Mistakes:

• Placing EXPLAIN after SELECT or at the end
• Using EXPLAIN as a function inside SELECT
• Writing EXPLAIN in the wrong order

3. Given the table employees(id, name, department_id) with an index on department_id, what does the type column in EXPLAIN output usually show when the index is used?

medium

A. ALL

B. index

C. ref

D. const

Solution

1. Step 1: Understand EXPLAIN 'type' values

   The 'type' column shows how MySQL accesses rows. 'ALL' means full table scan, 'ref' means index lookup by reference, 'index' means full index scan, and 'const' means constant lookup.

2. Step 2: Identify index usage for a foreign key

   When searching by an indexed column like department_id, MySQL uses 'ref' to find matching rows efficiently.

3. Final Answer:

   ref -> Option C

4. Quick Check:

   Index lookup by reference = ref = B [OK]

Hint: 'ref' means index used for matching rows, not full scan [OK]

Common Mistakes:

• Confusing 'ALL' with index usage
• Thinking 'index' means index lookup by key
• Assuming 'const' applies to indexed foreign keys

4. You run EXPLAIN SELECT * FROM orders WHERE customer_id = 5; but notice the type column shows ALL. What is the most likely reason?

medium

A. There is no index on customer_id

B. The query syntax is incorrect

C. The table is empty

D. MySQL automatically ignores indexes

Solution

1. Step 1: Interpret 'ALL' in EXPLAIN output

   'ALL' means MySQL is scanning the entire table, which is slow and usually means no index is used.

2. Step 2: Identify cause of full table scan

   If the query filters by customer_id but no index exists on that column, MySQL must scan all rows.

3. Final Answer:

   There is no index on customer_id -> Option A

4. Quick Check:

   Full scan means missing index = A [OK]

Hint: 'ALL' means full scan, usually missing index on filter column [OK]

Common Mistakes:

• Assuming syntax error causes 'ALL'
• Thinking empty table causes 'ALL'
• Believing MySQL ignores indexes by default

5. You have a query joining products and categories tables on category_id. EXPLAIN shows type as ALL for products and ref for categories. What is the best way to improve performance?

hard

A. Add an index on products.category_id

B. Rewrite the query without JOIN

C. Add an index on categories.category_id

D. Use SELECT * instead of specific columns

Solution

1. Step 1: Analyze EXPLAIN output for join

   'ALL' on products means full scan; 'ref' on categories means index used there.

2. Step 2: Identify missing index

   Since products is scanned fully, adding an index on products.category_id helps MySQL find matching rows faster.

3. Step 3: Evaluate other options

   Rewriting query or adding index on categories.category_id won't help if products lacks index. SELECT * vs specific columns affects data size, not join efficiency.

4. Final Answer:

   Add an index on products.category_id -> Option A

5. Quick Check:

   Full scan means missing index on join column = A [OK]

Hint: Add index on table with 'ALL' type to speed up joins [OK]

Common Mistakes:

• Adding index on already indexed table
• Removing JOIN instead of indexing
• Ignoring EXPLAIN 'type' column meaning