Clustered index vs Non clustered index structure

Clustered and Non clustered indexes are stored in a B – tree structure .

Clustered Index

• Clustered index enforce the logical order. (Misconception: Clustered index does not enforce the physical order)

• A table has only one clustered index because, the original table stored at the leaf level of the clustered index (Data pages).

• When you create a primary key by default clustered index will be created internally. (If the table has clustered index already then the non clustered index will be created internally)

• If the table does not have clustered index it’s called “Heap”

Non Clustered Index

• Non clustered indexes are separate storage. (I.e. original table and an index stored separately)

• The non clustered index does not enforce the logical order. The physical order of the rows is not the same as the index order.

• A table has 999 non clustered indexes in SQL-2008, 249 non clustered indexes prior to 2008.

• When you create a unique key by default non clustered index will be created internally.