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Clustered and non-clustered indexes are essential concepts in database management, each serving a different purpose. A clustered index helps sort data rows according to their key values, physically storing it in memory as ordered. On the contrary, a non-clustered index creates an organized, logical order for data rows and uses pointers to access physical data files.
A clustered index allows data pages to be stored within its leaf nodes, while non-clustered index methods never store such pages there. A clustered index is significantly larger in terms of size than a non-clustered index, while being much slower to access data. It also responds faster than the non-clustered index when it comes to the speed of accessing information, although both have their advantages.
Clustered indexes require no extra disk space, while non-clustered indexes need more disk space to store the index separately. By default, clustered indexes serve as primary keys of a table; Non-clustered indexes can only be used with an exclusive constraint on the table.
Clustered vs. Non-Clustered Index: Side-By-Side Comparison
| Parameters | Clustered | Non-clustered |
|---|---|---|
| Use for | Sorts records and stores them physically in memory according to their order | Creates a logical order for data rows and uses pointers to physical data files |
| Storing method | Stores data pages in the leaf nodes of the index | Never stores data pages in the leaf nodes of the index |
| Size | Large | Smaller compared to a clustered index |
| Data accessing | Faster than a non-clustered index | Slower compared to a clustered index |
| Additional disk space | Not required | Required to store the index separately |
| Type of key | By default, the primary keys of the table are a clustered index | Can be used with a unique constraint on the table that acts as a composite key |
| Main feature | Improves the performance of data retrieval | Should be created on columns that are used in joins |
Clustered vs. Non-Clustered Index: What’s the Difference?
Clustered and non-clustered indexes are two database indexing techniques to enhance query performance. However, the two techniques have differences that need to be taken into consideration. Fundamentally, clustered and non-clustered indexes differ by:
Data Storage Method
A clustered index stores data pages at the leaf nodes of the index, making it simpler for the database to retrieve it quickly. The pages are organized based on index keys which are sorted by default, making it simpler for the database to find desired information quickly.
Non-clustered indexes store key values only and use pointers to the actual data rows. Since this data is stored separately from the index, and its location cannot be determined from within it, database administrators must rely on pointers in order to locate it.
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Speed
A clustered index is faster than a non-clustered one because the data and index are stored together. This simplifies retrieving information from the database — use the index instead of pointers to access the desired data quickly.
Non-clustered indexes require extra processing to locate data, making them slower compared to clustered indexes. The database must use pointers within the index to locate it, taking extra time. Furthermore, having to retrieve from its separate location adds another step into this process; further extending retrieval times.
Memory Usage
Clustered indexes require less memory to run operations since both data and index are stored together. This enables databases to access information directly without using pointers, thus saving space by eliminating pointers altogether.
Non-clustered indexes require more memory to execute operations due to the additional processing needed to locate data. Databases must use pointers within the index, requiring additional space, while retrieving that data from its separate location necessitates even more memory.
Number of Indexes Per Table
Tables can only have one clustered index, as this determines the physical order of data in the table and can only be stored one way. A table with multiple clustered indexes would result in conflicting physical orders of data leading to consistency.
Tables can contain multiple non-clustered indexes since these don’t dictate the physical order of data in the table but instead create a logical hierarchy for rows. This enables multiple non-clustered indexes to coexist in one table without causing conflicts or disruption.
Disk Space Usage
Clustered indexes don’t need extra disk space since both data and index are stored together. Index keys are used to sort data, with data organized according to those keys. Therefore, there’s no need for extra space for storing the index separately.
Non-clustered indexes require more disk space to store them apart from data since the index contains pointers to its location. Since this data must be stored apart from the index, more space may be needed for storage if available disk space is limited.
Key Type
Table primary keys are typically used as clustered indexes. This index is created based on the primary key column and utilizes it for sorting data within the table, leading to faster retrieval times and improved performance.
Non-clustered indexes can be combined with a unique constraint on the table to serve as composite keys, providing secondary data access and improving query performance when using keys that aren’t assigned as primary keys. Furthermore, non-clustered indexes can be created on any column in the table – including those used in joins — to boost query speed further.
Fragmentation
Clustered indexes can become fragmented over time, which could significantly impact performance. Fragmentation occurs when data in a table is altered, leading to the disorganized physical order of that data in the database. As a result, queries must now jump back and forth between different parts of the disk for retrieval, leading to slower response times.
Non-clustered indexes do not become fragmented since they do not store the data physically in the index. Instead, the index contains pointers to where data resides, eliminating the need for it to maintain a physical order. This reduces fragmentation risks and provides better performance over time.
Operations
Clustered indexes offer two primary operations: clustered index scans and seeks. Clustered index scans retrieve all data contained within a table, while clustered index seeks to retrieve specific information based on index keys. Both operations provide fast data retrieval with improved performance.
Non-clustered indexes provide two types of operations: non-clustered index scans and non-clustered index seeks. Non-clustered index scans retrieve all data in a table, while non-clustered index seeks to retrieve specific data based on index keys. These operations enable better query performance when using keys that are not assigned as primary keys.
Size
Clustered indexes tend to be larger than non-clustered ones, as they store data and index together. As more data is added to a table, however, this could cause performance issues if disk space becomes limited.
Non-clustered indexes are smaller in size compared to clustered ones since they only store index keys and pointers to data locations, leading to a smaller overall size. Furthermore, since the non-clustered index does not increase in size as more data is added to the table, performance can be improved if disk space is limited.
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Clustered vs. Non-Clustered Index: 6 Must-Know Facts
- Clustered indexes organize data rows in a table based on key values, while non-clustered indexes store data at one location and indexes at another.
- Clustered indexes provide faster data access than non-clustered ones do; however, non-clustered ones require more processing power for faster response times.
- Clustered indexes require less memory to execute operations, while non-clustered indexes need more.
- Clustered indexes store data pages within their leaf nodes, while non-clustered ones don’t.
- A single table can only have one clustered index but multiple non-clustered ones.
- Clustered indexes offer the advantage of storing data on disk, while non-clustered indexes do not.
Clustered vs. Non-Clustered Index: Which One Is Better? Which One Should You Use?
Clustered and non-clustered indexes are two different indexing techniques used in SQL to enhance data retrieval performance. A clustered index sorts rows according to key values, and stores pages as leaf nodes of the index, while a non-clustered index stores data at one location and indexes at another; it includes pointers to these locations where relevant.
Clustered indexes offer the advantage of storing data and index together, making them faster than Non-clustered indices that require more memory for operations. On the contrary, Non-clustered indexes require more resources to run operations and never save data sheets in their leaf nodes.
Clustered indexes are usually the primary key of a table, while non-clustered indexes may exist in multiple locations. When combined with unique constraints on the table, these non-clustered indexes act as composite keys.
Finally, the choice of index depends on the data’s nature and purpose. Clustered indexes are ideal for improving data retrieval performance, while non-clustered indexes should be created on columns used in joins. Clustered indexes are larger but faster and require no extra disk space, while non-clustered indexes require extra disk space but have smaller sizes with slower speeds.
