Index maintenance in MySQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of index maintenance in a database?

Index maintenance ensures that indexes remain efficient and up-to-date, which helps speed up data retrieval and keeps the database performing well.

beginner

What happens if indexes are not maintained regularly?

Indexes can become fragmented or outdated, causing slower queries and increased resource use because the database has to work harder to find data.

intermediate

Which MySQL command can be used to rebuild an index?

You can use ALTER TABLE table_name DROP INDEX index_name, ADD INDEX index_name (columns); or OPTIMIZE TABLE table_name; to rebuild and defragment indexes.

intermediate

What is index fragmentation and why is it important to fix it?

Fragmentation means the index pages are not stored in a continuous order, which slows down data access. Fixing it improves speed and reduces disk I/O.

intermediate

How does the OPTIMIZE TABLE command help with index maintenance in MySQL?

It reorganizes the physical storage of table data and associated indexes, reducing fragmentation and reclaiming unused space to improve query speed.

What is the effect of index fragmentation on database performance?

A No effect at all

B Slower query response times

C More efficient memory usage

D Faster data insertion

Fragmented indexes cause slower queries because the database reads scattered data pages.

Which MySQL command can help reduce index fragmentation?

A OPTIMIZE TABLE

B CREATE INDEX

C DROP TABLE

D SHOW INDEX

OPTIMIZE TABLE reorganizes data and indexes to reduce fragmentation.

Why should you maintain indexes regularly?

A To disable indexing

B To delete all data

C To increase disk space usage

D To keep queries fast and efficient

Regular maintenance keeps indexes efficient, improving query speed.

Which of these is NOT a method of index maintenance?

A Dropping unused indexes

B Defragmenting indexes

C Increasing table size

D Rebuilding indexes

Increasing table size is unrelated to index maintenance.

What does the ALTER TABLE command do in index maintenance?

A Rebuilds or modifies indexes

B Deletes the entire table

C Shows table structure

D Backs up the database

ALTER TABLE can drop and recreate indexes to rebuild them.

Explain why index maintenance is important for database performance.

Think about how data is stored and accessed.

Describe common methods to maintain indexes in MySQL.

Focus on commands and practices.

Practice

1. What is the main purpose of maintaining indexes in a MySQL database?

easy

A. To speed up data searches and queries

B. To increase the size of the database

C. To delete all data from tables

D. To backup the database automatically

Solution

1. Step 1: Understand what indexes do

   Indexes help the database find data faster, like an index in a book.

2. Step 2: Identify the main benefit of index maintenance

   Maintaining indexes keeps searches quick and efficient.

3. Final Answer:

   To speed up data searches and queries -> Option A

4. Quick Check:

   Indexes improve search speed = D [OK]

Hint: Indexes speed up queries by organizing data [OK]

Common Mistakes:

• Thinking indexes increase database size only
• Confusing index maintenance with data backup
• Believing indexes delete data

2. Which of the following is the correct syntax to drop an index named idx_name from a table users in MySQL?

easy

A. DELETE INDEX idx_name FROM users;

B. DROP INDEX idx_name ON users;

C. REMOVE INDEX idx_name IN users;

D. DROP idx_name FROM users;

Solution

1. Step 1: Recall MySQL syntax for dropping an index

   The correct command is DROP INDEX index_name ON table_name;.

2. Step 2: Match the syntax with options

   DROP INDEX idx_name ON users; matches the correct syntax exactly.

3. Final Answer:

   DROP INDEX idx_name ON users; -> Option B

4. Quick Check:

   DROP INDEX ... ON ... is correct syntax = B [OK]

Hint: Use DROP INDEX index_name ON table_name; to remove indexes [OK]

Common Mistakes:

• Using DELETE or REMOVE instead of DROP
• Omitting ON keyword
• Wrong keyword order

3. Given the table products with an index on category_id, what will be the effect of running OPTIMIZE TABLE products;?

medium

A. It renames the products table

B. It deletes all rows from the products table

C. It adds a new index on category_id automatically

D. It rebuilds the table and its indexes to improve performance

Solution

1. Step 1: Understand what OPTIMIZE TABLE does

   OPTIMIZE TABLE rebuilds the table and its indexes to reduce fragmentation and improve speed.

2. Step 2: Analyze the options

   Only It rebuilds the table and its indexes to improve performance correctly describes this effect.

3. Final Answer:

   It rebuilds the table and its indexes to improve performance -> Option D

4. Quick Check:

   OPTIMIZE TABLE rebuilds table and indexes = A [OK]

Hint: OPTIMIZE TABLE rebuilds indexes and table for better speed [OK]

Common Mistakes:

• Thinking OPTIMIZE deletes data
• Assuming it adds indexes automatically
• Confusing it with renaming tables

4. You tried to drop an index using DROP INDEX idx_user ON users; but got an error. What is a likely cause?

medium

A. The table users does not exist

B. You must use DELETE INDEX instead of DROP INDEX

C. The index name idx_user does not exist on the table

D. You need to restart the MySQL server before dropping indexes

Solution

1. Step 1: Check common reasons for DROP INDEX errors

   An error usually occurs if the index name is wrong or missing.

2. Step 2: Evaluate the options

   The index name idx_user does not exist on the table is the most common cause; other options are incorrect commands or unnecessary actions.

3. Final Answer:

   The index name idx_user does not exist on the table -> Option C

4. Quick Check:

   Wrong index name causes DROP INDEX error = C [OK]

Hint: Verify index name exists before dropping it [OK]

Common Mistakes:

• Using DELETE instead of DROP
• Assuming server restart is needed
• Not checking table existence first

5. You have a large table orders with a composite index on (customer_id, order_date). After frequent inserts and deletes, queries on customer_id are slow. What is the best index maintenance step to improve performance?

hard

A. Run OPTIMIZE TABLE orders; to rebuild the table and indexes

B. Drop the composite index and create separate indexes on each column

C. Delete all rows and reload the table

D. Rename the table to refresh indexes

Solution

1. Step 1: Understand index fragmentation impact

   Frequent inserts and deletes cause index fragmentation, slowing queries.

2. Step 2: Choose the best maintenance action

   OPTIMIZE TABLE rebuilds indexes and reduces fragmentation, improving query speed.

3. Step 3: Evaluate other options

   Dropping and recreating indexes separately may not help and can be costly; deleting data or renaming table is unrelated.

4. Final Answer:

   Run OPTIMIZE TABLE orders; to rebuild the table and indexes -> Option A

5. Quick Check:

   OPTIMIZE TABLE fixes fragmentation = A [OK]

Hint: Use OPTIMIZE TABLE to fix fragmented indexes after many changes [OK]

Common Mistakes:

• Dropping composite index without reason
• Thinking renaming table refreshes indexes
• Deleting data instead of maintaining indexes