SQL Server heap v.s. clustered index

Question

I am using SQL Server 2008. I know if a table has no clustered index, then it is called heap, or else the storage model is called clustered index (B-Tree).

I want to lea

Answer 1

Heap storage has nothing to do with these heaps.

Heap just means records themselves are not ordered (i. e. not linked to one another).

When you insert a record, it just gets inserted into the free space the database finds.

Updating a row in a heap based table does not affect other records (though it affects secondary indexes)

If you create a secondary index on a HEAP table, the RID (a kind of a physical pointer to the storage space) is used as a row pointer.

Clustered index means that the records are part of a B-Tree. When you insert a record, the B-Tree needs to be relinked.

Updating a row in a clustered table causes relinking of the B-Tree, i. e. updating internal pointers in other records.

If you create a secondary index on a clustered table, the value of the clustered index key is used as a row pointer.

This means a clustered index should be unique. If a clustered index is not unique, a special hidden column called uniquifier is appended to the index key that makes if unique (and larger in size).

It is also worth noting that creating a secondary index on a column makes the values or the clustered index's key to be the part of the secondayry index's key.

By creating an index on a clustered table, you in fact always get a composite index

CREATE UNIQUE CLUSTERED INDEX CX_mytable_1234 (col1, col2, col3, col4)

CREATE INDEX IX_mytable_5678 (col5, col6, col7, col8)

Index IX_mytable_5678 is in fact an index on the following columns:

col5
col6
col7
col8
col1
col2
col3
col4

This has one more side effect:

A DESC condition in a single-column index on a clustered table makes sense in SQL Server

This index:

CREATE INDEX IX_mytable ON mytable (col1)

can be used in a query like this:

SELECT  TOP 100 *
FROM    mytable
ORDER BY
       col1, id

, while this one:

CREATE INDEX IX_mytable ON mytable (col1 DESC)

can be used in a query like this:

SELECT  TOP 100 *
FROM    mytable
ORDER BY
       col1, id DESC