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

SQLeasy0.4 min read

**Architectural Logic**: Clustered: Data stored in index order; one per table (usually PK). Table = clustered index. Non-clustered: Separate structure with pointers to data; multiple per table. **Why**: Clustered optimizes range scans (e.g., ORDER BY PK); inserts may cause page...

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Frequency
Low
Asked at 2 companies
Category
487
questions in SQL
Difficulty Split
130E|271M|86H
in this category
Total Bank
1,863
across 7 categories
Asked at these companies
PresidioSwiggy
Key Concepts Tested
bigquerysql

Why This Question Matters

This easy-level SQL question appears frequently in data engineering interviews at companies like Presidio, Swiggy. While less common, it tests deeper understanding that distinguishes strong candidates. Mastering the underlying concepts (bigquery, sql) will help you answer variations of this question confidently.

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Start by clearly defining the core concept being asked about. Interviewers want to see that you understand the fundamentals before diving into implementation details. Structure your answer with a definition, then explain the practical application with a concise example.

Expert Answer
82 words

Architectural Logic: Clustered: Data stored in index order; one per table (usually PK). Table = clustered index. Non-clustered: Separate structure with pointers to data; multiple per table. Why: Clustered optimizes range scans (e.g., ORDER BY PK); inserts may cause page splits. Non-clustered optimizes lookups and covering queries. Scalability: Clustered on wrong key can hurt insert performance. Non-clustered adds write amplification. Cost: Clustered has no extra storage; non-clustered duplicates key columns. Note: BigQuery/Redshift are columnar—no clustered/non-clustered; this is row-store (SQL Server, MySQL) specific.

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