Clustered and non-clustered indexes are two commonly used index types in database management systems, differing fundamentally in data storage and retrieval methods. The key differences are as follows:
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Data Storage Method:
- Clustered Index: In a clustered index, table rows are physically stored on disk in the order of the index key. This means a table can have only one clustered index, as data is stored in a single physical order.
- Non-Clustered Index: In a non-clustered index, the index structure is separate from the physical storage of the table. The index contains pointers to the rows in the data table, which can be stored in any order on disk. Therefore, a table can have multiple non-clustered indexes.
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Query Performance:
- Clustered Index: Because data rows are stored in the same physical order as the index, queries using a clustered index are highly efficient. Especially for range queries, as the data is stored sequentially.
- Non-Clustered Index: Query performance may be less efficient compared to clustered indexes because each query requires locating the data row pointer via the index and then accessing the actual data row, involving two addressing steps.
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Impact of Insertion and Modification:
- Clustered Index: Since data must be stored in the order of the index, inserting new data or modifying data that alters the data order may result in physical reorganization of the data, affecting performance.
- Non-Clustered Index: Insert and update operations have minimal impact because these operations do not affect the physical order of data in the table. Only the pointers in the index need to be updated accordingly.
Example: Consider a student table with columns for student IDs, names, and scores. If we create a clustered index on the student ID, the student records will be stored on disk in ID order. Querying student information by ID allows for rapid data retrieval.
Conversely, if we create a non-clustered index on the student scores, the index contains the scores and pointers to the student records. When querying students with a specific score, the database first searches the non-clustered index and then accesses the actual student records via the pointers in the index. In this case, the score index does not affect the physical storage order of the table data.
Summary: The choice between clustered and non-clustered indexes depends on specific application scenarios and query needs. Understanding these differences can help optimize database performance during database design and index selection.