SQL Server Indexes: Ascending or Descending? When Should You Choose Which?

SQL Server Index: Ascending or Descending?

When creating an index in SQL Server, you can choose whether the index is in ascending or descending order. This setting is often overlooked, but plays a vital role when working with composite indexes.

Composite Index

Compound indexes span multiple columns and can be used for sorting in a variety of ways. For example, consider the following index:

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

This index can efficiently handle the following queries:

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

SELECT  *
FROM    mytable
ORDER BY
        col1 DESC, col2;</code>

However, it cannot accommodate queries that require sorting col1 and col2 in ascending order:

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

Single column index

For single-column indexes, the selection of ascending and descending order is less meaningful. However, an edge case arises when dealing with aggregated tables (tables whose rows are physically ordered based on a specific key).

In these tables, if you create a descending index on a non-key column, it will affect the internal ordering of the data. This may cause performance issues when querying cluster keys in descending order:

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

In summary, the choice of ascending and descending indexes depends on the expected query pattern. For composite indexes, it ensures optimal performance for specific sorting requirements. For single-column indexes in clustered tables, it affects the efficiency of queries involving non-key columns and clustered keys.

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