When it comes to optimizing database performance, indexes play a crucial role. They are used to improve query performance by allowing faster access and retrieval of data. In this blog post, we will explore the difference between two commonly used types of indexes: clustered and non-clustered indexes.
Clustered Indexes
A clustered index determines the physical order of data within a table. Each table can have only one clustered index. It defines the order in which data is stored on disk, based on the values of one or more columns. It is important to note that a clustered index physically rearranges the data on disk, making it more efficient for certain types of queries.
Key points about clustered indexes:
- A clustered index is created on a table's primary key by default. If a primary key does not exist, a unique key can be used instead.
- The order of the data in the table matches the order of the clustered index.
- When a table has a clustered index, it means that the data is physically stored in the order of the index.
- Clustered indexes are ideal for tables with large amounts of data that are frequently accessed in a specific order.
- The data in a clustered index can be both sorted and searched at the same time.
Non-Clustered Indexes
A non-clustered index is a data structure that improves the performance of data retrieval operations. Unlike clustered indexes, non-clustered indexes do not dictate the physical order of data on disk. Instead, they create a separate structure that points to the physical location of the data.
Key points about non-clustered indexes:
- A table can have multiple non-clustered indexes.
- Non-clustered indexes are created on columns other than the primary or unique key.
- Non-clustered indexes improve the performance of queries that do not directly match the order of the data in the table.
- A non-clustered index contains a copy of the indexed column values along with a pointer to the actual data.
Differences Between Clustered and Non-Clustered Indexes
Both clustered and non-clustered indexes have their own advantages and use cases. Here are some key differences:
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Data Ordering: Clustered indexes determine the order of data on disk, while non-clustered indexes do not have this functionality. This means that a clustered index can improve the performance of queries that retrieve data in the order specified by the index.
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Storage: Clustered indexes rearrange the data on disk, potentially requiring additional storage space. Non-clustered indexes, on the other hand, create separate structures that point to the actual data, resulting in a smaller footprint.
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Sorting and Searching: Clustered indexes allow for both sorting and searching simultaneously. Non-clustered indexes, however, only allow for searching.
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Unique Values: Clustered indexes can have unique values, meaning that each value in the index is unique. Non-clustered indexes can have duplicate values.
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Primary Key: A clustered index can be created on a primary key by default, but a non-clustered index must be explicitly created on a primary key or unique key.
In conclusion, both clustered and non-clustered indexes have their own unique purposes and advantages. Understanding the differences between them will help developers make more informed decisions when designing and optimizing database structures. Clustered indexes are great for large tables that are frequently accessed in a specific order, while non-clustered indexes are useful for improving the performance of queries that do not directly match the order of the data.
本文来自极简博客，作者：时间的碎片，转载请注明原文链接：Exploring the Difference Between Clustered
