A clustered index in sql server like a dictionary or telephone directory. A,B,C,D it goes on.
If you look up with a name Ozgur you start O then z etc.

I would like a simple explaination for this concept for oracle savy users.

More information about Clustered Indexes in Sql Server. What is a Clustered Index?

1 Answer 1


In Oracle terminology, it is an Index Organized Table, commonly referred to as an IOT.

An index-organized table has a storage organization that is a variant of a primary B-tree. Unlike an ordinary (heap-organized) table whose data is stored as an unordered collection (heap), data for an index-organized table is stored in a B-tree index structure in a primary key sorted manner. Each leaf block in the index structure stores both the key and nonkey columns.

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    It is probably the closest thing to a clustered index, but it sure isn't the same thing.
    – user1822
    Commented Nov 16, 2011 at 9:49
  • @a_horse_with_no_name How do they differ? Commented Nov 16, 2011 at 9:53
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    Hi horse! IN SQL Server the clustered index also is physically the table - the table data is physically stored in the index. Commented Nov 16, 2011 at 11:12
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    @a_horse_with_no_name, after reading on IOTs, it's exactly the same thing as clustered indexes in SQL Server: a B-Tree with the index keys, and sorted leaves by the index keys. download.oracle.com/docs/cd/E14072_01/server.112/e10713/…
    – gonsalu
    Commented Nov 16, 2011 at 11:57
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    @Gonsalu: but one of the differences is still that SQL Server can create a clustered index on non-unique (i.e PK) columns. Btw: I found this page quite interesting: use-the-index-luke.com/sql/clustering/…
    – user1822
    Commented Nov 16, 2011 at 13:32

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