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    <p>Heap storage has nothing to do with <a href="http://en.wikipedia.org/wiki/Heap_%28data_structure%29" rel="noreferrer">these heaps</a>.</p> <p>Heap just means records themselves are not ordered (i. e. not linked to one another).</p> <p>When you insert a record, it just gets inserted into the free space the database finds.</p> <h3>Updating a row in a heap based table does not affect other records (though it affects secondary indexes)</h3> <p>If you create a secondary index on a <code>HEAP</code> table, the <code>RID</code> (a kind of a physical pointer to the storage space) is used as a row pointer.</p> <p>Clustered index means that the records are part of a <code>B-Tree</code>. When you insert a record, the <code>B-Tree</code> needs to be relinked.</p> <h3>Updating a row in a clustered table causes relinking of the B-Tree, i. e. updating internal pointers in other records.</h3> <p>If you create a secondary index on a clustered table, the value of the clustered index key is used as a row pointer.</p> <p>This means a clustered index should be unique. If a clustered index is not unique, a special hidden column called <code>uniquifier</code> is appended to the index key that makes if unique (and larger in size).</p> <p>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.</p> <h3>By creating an index on a clustered table, you in fact always get a composite index</h3> <pre><code>CREATE UNIQUE CLUSTERED INDEX CX_mytable_1234 (col1, col2, col3, col4) CREATE INDEX IX_mytable_5678 (col5, col6, col7, col8) </code></pre> <p>Index <code>IX_mytable_5678</code> is in fact an index on the following columns:</p> <pre><code>col5 col6 col7 col8 col1 col2 col3 col4 </code></pre> <p>This has one more side effect:</p> <h3>A <code>DESC</code> condition in a single-column index on a clustered table makes sense in <code>SQL Server</code></h3> <p>This index:</p> <pre><code>CREATE INDEX IX_mytable ON mytable (col1) </code></pre> <p>can be used in a query like this:</p> <pre><code>SELECT TOP 100 * FROM mytable ORDER BY col1, id </code></pre> <p>, while this one:</p> <pre><code>CREATE INDEX IX_mytable ON mytable (col1 DESC) </code></pre> <p>can be used in a query like this:</p> <pre><code>SELECT TOP 100 * FROM mytable ORDER BY col1, id DESC </code></pre>
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    1. USQuassnoi
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    1. COSo, is inserting a row into a heap faster/slower/same as inserting a row into a B-Tree?
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      1. USPilotBob
    2. COInserting into a heap is *a little bit* faster than that to a `B-Tree` if no page split occurs on the latter, and *much* faster if the page split occurs (i. e. there is no place to enter the new row and the data should be redistributed between the leaves).
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    3. CODamn you're good, Quassnoi... you know how much garbage exists on the net regarding Clustered indexes...
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      1. USStephanie Page
 

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