Index selection strategy in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of using indexes in a database?

Indexes help the database find data faster, like a shortcut in a book's table of contents.

beginner

Which type of queries benefit most from indexes?

Queries that search for specific values or ranges, such as SELECT with WHERE clauses, benefit most from indexes.

intermediate

Why should you avoid indexing every column in a table?

Because indexes take extra space and slow down data changes like INSERT, UPDATE, and DELETE.

intermediate

What is a composite index?

An index that includes multiple columns to speed up queries filtering by those columns together.

advanced

How does MySQL choose which index to use for a query?

MySQL uses the query optimizer to pick the index that will return results fastest based on statistics and query conditions.

Which of these is NOT a good reason to create an index?

A Improving INSERT performance

B Helping with JOIN operations

C Speeding up SELECT queries

D Filtering rows faster in WHERE clauses

Indexes slow down INSERT operations because the index must be updated with new data.

What does a composite index contain?

A Multiple columns

B Only foreign keys

C Only primary keys

D One column only

A composite index includes multiple columns to optimize queries filtering by those columns together.

When should you consider adding an index to a column?

A When the column is rarely used in queries

B When the column stores large text blobs

C When the column is often used in WHERE clauses

D When the column is only used for sorting

Indexes help most when columns are frequently used to filter data in WHERE clauses.

What is a downside of having too many indexes?

A Queries run faster

B More disk space is used

C Data updates become slower

D Both B and C

Too many indexes use more disk space and slow down data changes like INSERT, UPDATE, and DELETE.

How does MySQL decide which index to use?

A Uses the first created index

B Uses the query optimizer based on statistics

C Randomly picks one

D Always uses the primary key index

MySQL's query optimizer chooses the best index based on data statistics and query conditions.

Explain how to decide which columns to index in a MySQL table.

Think about which columns help find data quickly and which slow down updates.

Describe the trade-offs involved in adding indexes to a database table.

Indexes are like shortcuts but cost extra to maintain.

Practice

1. What is the main purpose of using an index in a MySQL database?

easy

A. To increase the size of the database

B. To speed up data retrieval by creating shortcuts

C. To backup the database automatically

D. To store data in a compressed format

Solution

1. Step 1: Understand what an index does

   An index creates a shortcut to quickly find data without scanning the whole table.

2. Step 2: Compare options with index purpose

   Only To speed up data retrieval by creating shortcuts describes speeding up data retrieval, which is the main use of indexes.

3. Final Answer:

   To speed up data retrieval by creating shortcuts -> Option B

4. Quick Check:

   Index = Speed up search [OK]

Hint: Indexes speed up searches by creating shortcuts [OK]

Common Mistakes:

• Thinking indexes compress data
• Confusing indexes with backups
• Believing indexes increase database size

2. Which of the following is the correct syntax to create an index on the column email in a table users?

easy

A. CREATE INDEX email_index ON users (email);

B. CREATE INDEX ON users email_index (email);

C. CREATE INDEX users.email_index (email);

D. INDEX CREATE email_index ON users (email);

Solution

1. Step 1: Recall correct CREATE INDEX syntax

   The correct syntax is: CREATE INDEX index_name ON table_name (column_name);

2. Step 2: Match syntax with options

   CREATE INDEX email_index ON users (email); matches the correct syntax exactly, others have wrong order or keywords.

3. Final Answer:

   CREATE INDEX email_index ON users (email); -> Option A

4. Quick Check:

   CREATE INDEX index_name ON table (column) [OK]

Hint: Remember: CREATE INDEX name ON table (column) [OK]

Common Mistakes:

• Swapping index name and table name
• Omitting ON keyword
• Wrong keyword order

3. Given a table orders with an index on customer_id, what will happen when you run this query?

SELECT * FROM orders WHERE customer_id = 123;

medium

A. The query will fail with a syntax error

B. The query will scan the entire table ignoring the index

C. The query will use the index to quickly find matching rows

D. The query will update the customer_id values

Solution

1. Step 1: Understand index usage in WHERE clause

   When a column in WHERE has an index, MySQL uses it to find rows faster.

2. Step 2: Analyze query behavior

   The query filters by customer_id, which has an index, so it uses the index to speed up search.

3. Final Answer:

   The query will use the index to quickly find matching rows -> Option C

4. Quick Check:

   Indexed column in WHERE = index used [OK]

Hint: WHERE on indexed column uses index automatically [OK]

Common Mistakes:

• Assuming full table scan always happens
• Confusing SELECT with UPDATE
• Expecting syntax errors in correct queries

4. You created an index on last_name but your query SELECT * FROM employees WHERE first_name = 'John'; is slow. What is the likely problem?

medium

A. The index on last_name is not used because the query filters on first_name

B. The index on last_name causes a syntax error

C. The query is missing a JOIN clause

D. The index automatically updates first_name values

Solution

1. Step 1: Identify which column is indexed

   The index is on last_name, but the query filters on first_name.

2. Step 2: Understand index usage

   Indexes only help if the filtered column is indexed; here first_name is not indexed, so index is unused.

3. Final Answer:

   The index on last_name is not used because the query filters on first_name -> Option A

4. Quick Check:

   Index column ≠ filtered column = no index use [OK]

Hint: Index helps only if filtered column matches indexed column [OK]

Common Mistakes:

• Expecting index on different column to help
• Thinking index causes syntax errors
• Confusing filtering with updating

5. You have a table products with columns category, price, and stock. You often run this query:

SELECT * FROM products WHERE category = 'Books' AND price < 20;

Which index strategy is best to speed up this query?

hard

A. Create an index only on price

B. Create separate indexes on category and stock

C. Create a composite index on (stock, price)

D. Create a composite index on (category, price)

Solution

1. Step 1: Analyze query filter columns

   The query filters on category and price together.

2. Step 2: Choose index covering both columns

   A composite index on (category, price) helps MySQL filter efficiently on both conditions.

3. Step 3: Evaluate other options

   Separate indexes or indexes on stock do not help this query's WHERE clause.

4. Final Answer:

   Create a composite index on (category, price) -> Option D

5. Quick Check:

   Composite index on filtered columns = best speed [OK]

Hint: Composite index on all filtered columns speeds queries best [OK]

Common Mistakes:

• Indexing unrelated columns
• Using separate indexes instead of composite
• Ignoring query filter columns