Clustered index
A clustered index is a database index whose key determines the physical order of rows in a table or indexed view.
100 SQL Server Mistakes and How to Avoid Them
A clustered index is an index whose key determines the physical order of rows in a table or indexed view.
- How it is used
- Frames clustered-key selection as a database-design decision affecting storage and lookup efficiency.
- How deep it goes
- Explains B-tree levels, leaf data pages, and seek-versus-scan page reads, while deferring fuller index discussion.
- Examples
- Uses SalesOrderHeaders and an NCHAR(12) order number to show the cost of a wide clustered key.
- What you can do after
- Evaluate/understand when a clustered key supports efficient reads and why a wide primary key can be costly.
Read how 100 SQL Server Mistakes and How to Avoid Them explains clustered index →
Learn SQL in a Month of Lunches
A clustered index is a table-organizing structure that determines the physical order in which rows are stored on a storage medium.
- How it is used
- Organizes table rows physically so commonly used retrieval, filtering, and joining operations can avoid unrestricted row scans.
- How deep it goes
- Explains physical ordering, scan implications, key selection, and table identity, but does not provide clustered-index creation syntax.
- Examples
- Uses a sorted telephone book and the MySQL shipment table’s PRIMARY index to illustrate physical row organization.
- What you can do after
- Evaluate clustered-index choices by considering common query columns, primary-key joins, physical ordering, and table-wide scan costs.
Read how Learn SQL in a Month of Lunches explains clustered index →
Related concepts
- indexingkind of · Indexing is the organization or use of positions, keys, or lookup structures to identify and retrieve selected data or elements.
- nonclustered indexcontrasts with · A nonclustered index is a separate database structure that stores keys and row locators without determining a table’s physical row order.
- primary keyproduced by · A primary key is a designated column or attribute, or combination of them, that uniquely identifies each row or item in a table or data store.
- B-treeimplements · A B-tree is a balanced multiway search tree that maintains ordered keys and supports efficient searching, insertion, deletion, and sequential access.
- database tablepart of · A database table is a relational database structure that organizes records into rows and defined columns.
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