SQL Server Indexes: Ascending or Descending? When Does Sort Order Matter?

SQL Server Index: Ascending or Descending - Revealing Its Importance

When creating an index in Microsoft SQL Server, you can choose to specify the order of the index columns as ascending or descending. While this may seem like a trivial choice, it can have significant consequences, especially in the context of compound indexes.

For example, consider the following compound index:

<code class="language-sql">CREATE INDEX ix_index ON mytable (col1, col2 DESC);</code>

This index can be used for the following queries:

<code class="language-sql">SELECT  *
FROM    mytable
ORDER BY
        col1, col2 DESC</code>

or

<code class="language-sql">SELECT  *
FROM    mytable
ORDER BY
        col1 DESC, col2</code>

However, it does not work effectively with:

<code class="language-sql">SELECT  *
FROM    mytable
ORDER BY
        col1, col2</code>

This asymmetry highlights the critical role of descending indexes in composite indexes. They enable efficient retrieval of data in a specific sort order that would otherwise require additional sorting operations.

Even for a single column index, the choice of ascending and descending order can be critical. In a clustered table where the data is physically sorted by the primary key, an ascending index on a nonprimary key column allows queries that sort the data by that column to be executed without additional sorting. Conversely, a descending index on the same column is less efficient for queries that require a descending sort order.

Essentially, the ascending or descending nature of an index determines the sort order it can be utilized most efficiently. By carefully considering potential query patterns, you can optimize performance by choosing the appropriate index type.

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