Ascending vs. Descending Indexes in SQL Server: When Does Order Matter?

SQL Server Indexes: Ascending vs. Descending – When Does Order Matter?

Creating indexes in Microsoft SQL Server often involves deciding between ascending and descending order. While binary search suggests order shouldn't affect lookup speed, certain situations highlight the importance of this choice.

Composite Indexes: Order's Significance

Consider a composite index:

CREATE INDEX ix_index ON mytable (col1, col2 DESC);

This index efficiently supports queries like:

• SELECT * FROM mytable ORDER BY col1, col2 DESC
• SELECT * FROM mytable ORDER BY col1 DESC, col2

However, it's less efficient for:

• SELECT * FROM mytable ORDER BY col1, col2

Single-Column Indexes: Order's Irrelevance

For single-column indexes, ascending or descending order generally offers equivalent performance.

Clustered Indexes: Order's Crucial Role

Clustered indexes are affected by the index order. For example:

CREATE TABLE mytable (pk INT NOT NULL PRIMARY KEY, col1 INT NOT NULL);
CREATE INDEX ix_mytable_col1 ON mytable (col1);

If ix_mytable_col1 is ascending, the query:

SELECT col1, pk
FROM mytable
ORDER BY col1, pk

executes efficiently without extra sorting. But, if ix_mytable_col1 is descending, additional sorting is needed, hindering performance.

In summary, the choice between ascending and descending index order significantly impacts composite indexes and clustered tables. The optimal order depends directly on the specific queries and data structure.

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