Primary vs secondary indexes in DBMS Theory - Quick Revision & Key Differences

Recall & Review

beginner

What is a primary index in a database?

A primary index is an index based on the primary key of a table. It uniquely identifies each record and is used to quickly locate data. It is usually automatically created when a primary key is defined.

beginner

What is a secondary index in a database?

A secondary index is an index created on non-primary key columns. It helps speed up queries on those columns but does not enforce uniqueness or directly identify records.

intermediate

How does a primary index differ from a secondary index?

A primary index is based on the primary key and uniquely identifies records, while a secondary index is based on other columns and may not be unique. Primary indexes are often clustered, meaning data is stored in the order of the index, but secondary indexes are usually non-clustered.

beginner

Why are secondary indexes useful?

Secondary indexes allow faster searching and retrieval of data based on columns other than the primary key. They improve query performance when filtering or sorting by those columns.

beginner

Can a table have multiple secondary indexes?

Yes, a table can have multiple secondary indexes on different columns to optimize various queries. However, each index adds some overhead for data updates.

Which index uniquely identifies each record in a table?

A Composite index

B Secondary index

C Primary index

D Foreign key index

The primary index is based on the primary key and uniquely identifies each record.

What is true about secondary indexes?

A They are based on non-primary key columns

B They are always unique

C They store data in sorted order

D They replace the primary index

Secondary indexes are created on columns other than the primary key and may not be unique.

Which of the following is a benefit of secondary indexes?

A Enforcing data uniqueness

B Speeding up queries on non-primary key columns

C Reducing storage space

D Automatically updating primary keys

Secondary indexes help speed up queries on columns other than the primary key.

Can a table have more than one secondary index?

A Yes, multiple secondary indexes can exist

B No, only one secondary index is allowed

C Only if the table has no primary key

D Only if the database is NoSQL

Tables can have multiple secondary indexes on different columns.

Which index type is usually clustered, storing data in index order?

A Hash index

B Secondary index

C Bitmap index

D Primary index

Primary indexes are often clustered, meaning the data is stored in the order of the index.

Explain the main differences between primary and secondary indexes in a database.

Think about uniqueness and which columns the indexes are built on.

Describe why secondary indexes are important and when you would use them.

Consider queries that filter by columns other than the primary key.

Practice

1. What is the main purpose of a primary index in a database?

easy

A. To provide unique and fast access to records using the primary key

B. To speed up searches on non-key columns

C. To store duplicate values for faster retrieval

D. To backup the database automatically

Solution

1. Step 1: Understand the role of primary index

   A primary index is created on the primary key of a table, which uniquely identifies each record.

2. Step 2: Identify its main function

   It ensures fast and unique access to records based on the primary key values.

3. Final Answer:

   To provide unique and fast access to records using the primary key -> Option A

4. Quick Check:

   Primary index = unique fast access [OK]

Hint: Primary index = unique key fast access [OK]

Common Mistakes:

• Confusing primary index with secondary index
• Thinking primary index allows duplicates
• Assuming primary index is for backup

2. Which of the following is the correct statement about creating a secondary index in SQL?

easy

A. CREATE INDEX idx_name ON table(column);

B. CREATE UNIQUE INDEX idx_name ON table(column);

C. CREATE PRIMARY INDEX idx_name ON table(column);

D. CREATE SECONDARY INDEX idx_name ON table(column);

Solution

1. Step 1: Recall SQL syntax for indexes

   Secondary indexes are created using the standard CREATE INDEX statement without the PRIMARY keyword.

2. Step 2: Identify the correct syntax

   CREATE INDEX idx_name ON table(column); uses the correct syntax: CREATE INDEX idx_name ON table(column);

3. Final Answer:

   CREATE INDEX idx_name ON table(column); -> Option A

4. Quick Check:

   Secondary index syntax = CREATE INDEX [OK]

Hint: Secondary index uses CREATE INDEX without PRIMARY [OK]

Common Mistakes:

• Using CREATE SECONDARY INDEX which is invalid
• Confusing with CREATE PRIMARY INDEX syntax
• Using UNIQUE keyword incorrectly for secondary index

3. Consider a table Employees(emp_id, name, department) where emp_id is the primary key. Which index type would speed up a query filtering by department?

medium

A. Primary index on department

B. Primary index on emp_id

C. Secondary index on department

D. No index needed

Solution

1. Step 1: Identify the primary key and its index

   The primary key is emp_id, so the primary index is on emp_id.

2. Step 2: Determine which index helps filter by department

   Since department is not the primary key, a secondary index on department speeds up queries filtering by it.

3. Final Answer:

   Secondary index on department -> Option C

4. Quick Check:

   Filter by non-key column = secondary index [OK]

Hint: Use secondary index for non-primary key columns [OK]

Common Mistakes:

• Assuming primary index helps filter by any column
• Trying to create primary index on non-key column
• Ignoring the benefit of secondary indexes

4. A developer created a secondary index on a column that contains many duplicate values. What is the likely problem?

medium

A. The database will reject the index creation

B. The primary index will be corrupted

C. The secondary index will enforce uniqueness

D. The secondary index will be inefficient due to low uniqueness

Solution

1. Step 1: Understand secondary index behavior with duplicates

   Secondary indexes can be created on columns with duplicates but may become less efficient because many records share the same key.

2. Step 2: Identify the impact on performance

   Low uniqueness means the index has many entries pointing to multiple rows, slowing down search performance.

3. Final Answer:

   The secondary index will be inefficient due to low uniqueness -> Option D

4. Quick Check:

   Duplicates in secondary index = inefficiency [OK]

Hint: Secondary index on duplicates slows searches [OK]

Common Mistakes:

• Thinking secondary index enforces uniqueness
• Believing primary index gets corrupted
• Expecting index creation to fail

5. You have a large table with a primary index on customer_id and a secondary index on city. You want to optimize queries filtering by both customer_id and city. What is the best indexing strategy?

hard

A. Drop the secondary index and rely only on primary index

B. Create a composite index on (customer_id, city)

C. Create a secondary index on customer_id only

D. Create two separate secondary indexes on customer_id and city

Solution

1. Step 1: Analyze current indexes and query filters

   Primary index exists on customer_id, secondary index on city. Queries filter by both columns.

2. Step 2: Understand composite index benefits

   A composite index on (customer_id, city) allows efficient filtering on both columns together, improving query speed.

3. Step 3: Evaluate other options

   Dropping indexes or creating separate secondary indexes won't optimize combined filtering as well as a composite index.

4. Final Answer:

   Create a composite index on (customer_id, city) -> Option B

5. Quick Check:

   Combined filter = composite index [OK]

Hint: Use composite index for multi-column filters [OK]

Common Mistakes:

• Dropping useful indexes
• Creating redundant secondary indexes
• Ignoring composite index advantages