Indexes speed the retrieval of data. Putting an index on a column often makes the difference between a quick response to a query and a long wait.
However, building an index takes time and storage space. For example, nonclustered indexes are automatically re-created when a clustered index is rebuilt.
Additionally, inserting, deleting, or updating data in indexed columns takes longer than in unindexed columns. However, this cost is usually outweighed by the extent to which indexes improve retrieval performance.
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