What is the difference between a clustered and non-clustered index?

 Theme: Database Performance  Role: Database Administrator  Function: Technology

  Interview Question for Database Administrator:  See sample answers, motivations & red flags for this common interview question. About Database Administrator: Manages and optimizes databases for efficient data storage and retrieval. This role falls within the Technology function of a firm. See other interview questions & further information for this role here

 Sample Answer 


  Example response for question delving into Database Performance with the key points that need to be covered in an effective response. Customize this to your own experience with concrete examples and evidence

  •  Definition: A clustered index determines the physical order of data in a table, while a non-clustered index does not
  •  Storage: A clustered index reorders the way data is stored on disk, while a non-clustered index creates a separate structure that points to the data
  •  Sorting: A clustered index automatically sorts the data in the table based on the indexed column(s), while a non-clustered index does not
  •  Number of Indexes: A table can have only one clustered index, but multiple non-clustered indexes
  •  Performance: A clustered index is generally faster for retrieving large ranges of data, while a non-clustered index is faster for retrieving specific rows or smaller ranges of data
  •  Data Modification: When a clustered index is modified, the entire table may need to be reorganized, while modifying a non-clustered index only affects the index structure
  •  Unique Constraint: A clustered index can enforce a unique constraint on a column, while a non-clustered index can also enforce a unique constraint, but it does not determine the physical order of data
  •  Primary Key: A clustered index is often used for the primary key of a table, while a non-clustered index can be used for primary key as well as other columns

 Underlying Motivations 


  What the Interviewer is trying to find out about you and your experiences through this question

  •  Technical knowledge: Assessing the candidate's understanding of database indexing and their ability to differentiate between clustered and non-clustered indexes
  •  Problem-solving skills: Evaluating the candidate's ability to analyze and optimize database performance by utilizing appropriate indexing strategies
  •  Experience & expertise: Determining the candidate's level of experience and expertise in managing and optimizing database indexes

 Potential Minefields 


  How to avoid some common minefields when answering this question in order to not raise any red flags

  •  Lack of understanding: Providing incorrect or vague definitions of clustered and non-clustered indexes
  •  Confusion between concepts: Mixing up the concepts of clustered and non-clustered indexes, or failing to explain the key differences between them
  •  Inability to explain use cases: Not being able to articulate the scenarios where a clustered or non-clustered index would be beneficial
  •  Limited knowledge of performance impact: Failing to discuss the performance implications of using clustered and non-clustered indexes
  •  Lack of familiarity with database management systems: Not being able to relate the concepts of clustered and non-clustered indexes to specific database management systems like SQL Server or Oracle

 Related

  Other questions asked for the Database Administrator in Technology function. View details for the Database Administrator here