When and Why Sort Direction in Database Indexes Actually Matters

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For single-column queries, databases like PostgreSQL, MySQL, and SQL Server can scan a B-Tree index both forward and backward with equal speed, making explicit sort direction in the index definition unnecessary. The primary benefit of indexing a sorted column is eliminating the costly filesort step, as rows are returned directly in order from the index structure. Problems arise with composite indexes when a query mixes sort directions, such as ordering one column ascending and another descending. A default composite index cannot efficiently serve mixed-direction queries, requiring the index definition to exactly match the query's ORDER BY directions. Best practice is to use a plain index for single-column sorts and reserve direction-specific composite indexes only for queries with mixed sorting requirements.

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