Composite index and column order in SQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a composite index in a database?

A composite index is an index that includes more than one column from a table. It helps speed up queries that filter or sort by multiple columns together.

intermediate

Why does the order of columns matter in a composite index?

The order matters because the database uses the index starting from the first column. Queries filtering on the first column(s) can use the index efficiently, but if the first column is missing in the query, the index might not be used.

intermediate

Given a composite index on (A, B), which query can use the index efficiently?

A query filtering on column A alone or on both A and B can use the index efficiently. But a query filtering only on B cannot use the index effectively.

beginner

How does a composite index improve query performance?

It allows the database to quickly find rows matching multiple column conditions without scanning the whole table, reducing search time.

intermediate

What happens if you reverse the column order in a composite index?

Reversing the order changes which queries can use the index efficiently. For example, an index on (B, A) helps queries filtering on B first, but not those filtering only on A.

What is a composite index?

A A type of table

B An index on a single column

C An index on multiple columns

D A database user

A composite index includes more than one column to speed up queries involving those columns.

If you have a composite index on (A, B), which query can use it efficiently?

A WHERE C = 5

B WHERE B = 5

C WHERE B = 5 AND A = 10

D WHERE A = 5

The index can be used efficiently when filtering on the first column A or both A and B, but not on B alone.

Why does column order matter in a composite index?

A Because the database uses the index starting from the first column

B Because the last column is always ignored

C Because columns are sorted alphabetically

D Because indexes only work on one column

The database uses the index starting from the first column, so queries must filter on that column to use the index.

What is the main benefit of using a composite index?

A Faster queries on multiple columns

B More storage space

C Slower inserts

D Less data in the table

Composite indexes speed up queries that filter or sort by multiple columns.

If you have an index on (B, A), which query is less likely to use it efficiently?

A WHERE B = 10

B WHERE A = 10

C WHERE B = 10 AND A = 20

D WHERE B > 5

Queries filtering only on A cannot use an index starting with B efficiently.

Explain what a composite index is and why the order of columns matters.

Think about how the database searches using the index.

Describe how a composite index can improve query performance and give an example of when it might not be used.

Consider which columns the query filters on compared to the index order.

Practice

1. What is the main benefit of using a composite index in a database?

easy

A. It encrypts the data for security.

B. It reduces the size of the database file.

C. It automatically backs up the database.

D. It speeds up queries filtering on multiple columns together.

Solution

1. Step 1: Understand what a composite index does

   A composite index is created on multiple columns to help the database find rows faster when queries filter on those columns together.

2. Step 2: Compare options to the definition

   Only It speeds up queries filtering on multiple columns together. correctly describes the benefit of speeding up queries using multiple columns. Other options describe unrelated features.

3. Final Answer:

   It speeds up queries filtering on multiple columns together. -> Option D

4. Quick Check:

   Composite index = speeds up multi-column queries [OK]

Hint: Composite index helps multi-column filters run faster [OK]

Common Mistakes:

• Thinking it reduces database size
• Confusing indexing with backup or encryption
• Assuming it speeds up single-column queries only

2. Which of the following is the correct syntax to create a composite index on columns last_name and first_name in a table employees?

easy

A. CREATE INDEX idx_name ON employees last_name, first_name;

B. CREATE INDEX idx_name ON employees (last_name, first_name);

C. CREATE INDEX idx_name (last_name, first_name) ON employees;

D. CREATE INDEX idx_name ON employees WHERE last_name, first_name;

Solution

1. Step 1: Recall correct CREATE INDEX syntax

   The correct syntax is: CREATE INDEX index_name ON table_name (column1, column2);

2. Step 2: Match options to syntax

   CREATE INDEX idx_name ON employees (last_name, first_name); matches the correct syntax exactly. Options A, C, and D have syntax errors or misplaced keywords.

3. Final Answer:

   CREATE INDEX idx_name ON employees (last_name, first_name); -> Option B

4. Quick Check:

   CREATE INDEX ... ON table (columns) [OK]

Hint: Use parentheses around columns after ON table [OK]

Common Mistakes:

• Omitting parentheses around columns
• Placing columns before ON keyword
• Using WHERE instead of column list

3. Given a composite index on (city, age) in a table users, which query will use the index efficiently?

medium

A. SELECT * FROM users WHERE age = 25 AND city = 'Boston';

B. SELECT * FROM users WHERE age > 30;

C. SELECT * FROM users WHERE city = 'Boston' AND age > 30;

D. SELECT * FROM users WHERE city = 'Boston' ORDER BY age DESC;

Solution

1. Step 1: Understand index column order usage

   A composite index on (city, age) can be used efficiently if the query filters on city first, then age.

2. Step 2: Analyze each query

   SELECT * FROM users WHERE city = 'Boston' AND age > 30; filters on city and age, matching index order. SELECT * FROM users WHERE age > 30; filters only on age (second column), so index is less useful. SELECT * FROM users WHERE age = 25 AND city = 'Boston'; filters age first, then city, which is less efficient. SELECT * FROM users WHERE city = 'Boston' ORDER BY age DESC; filters city but only orders by age, which may use index partially.

3. Final Answer:

   SELECT * FROM users WHERE city = 'Boston' AND age > 30; -> Option C

4. Quick Check:

   Index (city, age) best for city then age filters [OK]

Hint: Filter on first index column before others [OK]

Common Mistakes:

• Assuming index works well if filtering only second column
• Ignoring column order in composite index
• Thinking ORDER BY always uses index fully

4. You created a composite index on (department, salary) but your query filtering only on salary is slow. What is the likely cause?

medium

A. The index column order starts with department, so filtering only salary can't use it well.

B. The index is corrupted and needs rebuilding.

C. The salary column data type is incompatible with indexing.

D. The database does not support composite indexes.

Solution

1. Step 1: Recall how composite index column order affects usage

   Composite indexes work best when filtering starts with the first column. Filtering only on the second column (salary) won't use the index efficiently.

2. Step 2: Evaluate options

   The index column order starts with department, so filtering only salary can't use it well. correctly explains the cause. Options A, B, and D are unlikely or incorrect because the index exists and salary can be indexed normally.

3. Final Answer:

   The index column order starts with department, so filtering only salary can't use it well. -> Option A

4. Quick Check:

   Filter must start with first index column [OK]

Hint: Filter on first index column to use composite index [OK]

Common Mistakes:

• Assuming any column in index can be filtered efficiently alone
• Blaming index corruption without checking usage
• Ignoring column order importance

5. You have a table orders with columns customer_id, order_date, and status. You often run queries filtering by customer_id and sorting by order_date. Which composite index will optimize these queries best?

hard

A. CREATE INDEX idx_customer_date ON orders (customer_id, order_date);

B. CREATE INDEX idx_date_customer ON orders (order_date, customer_id);

C. CREATE INDEX idx_status_customer ON orders (status, customer_id);

D. CREATE INDEX idx_order ON orders (order_date);

Solution

1. Step 1: Identify query filter and sort columns

   Queries filter by customer_id and sort by order_date, so index should start with customer_id and include order_date next.

2. Step 2: Match index column order to query pattern

   CREATE INDEX idx_customer_date ON orders (customer_id, order_date); matches this order (customer_id, order_date), allowing efficient filtering and sorting. CREATE INDEX idx_date_customer ON orders (order_date, customer_id); reverses order, less efficient for filtering. Options A, B, and C do not match query needs.

3. Final Answer:

   CREATE INDEX idx_customer_date ON orders (customer_id, order_date); -> Option A

4. Quick Check:

   Index columns match filter then sort order [OK]

Hint: Index columns in filter then sort order [OK]

Common Mistakes:

• Reversing column order in composite index
• Indexing unrelated columns
• Using single-column index when composite needed