> For the complete documentation index, see [llms.txt](https://til.notdu.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://til.notdu.com/database/intro-to-database-systems/tree-indexes-i/cluster-indexes.md).

# Cluster indexes

The table is stored in the sort order specified by the primary key.

-> Can be ether heap- or index-organized storage.

Some DBMSs always use a clustered index.

-> If a table doesn't contain a primary key, the DBMS will automatically make a hidden row id primary key.

Other DBMSs cannot use them at all.