How Can I Efficiently Count Distinct Combinations Across Multiple Columns in SQL?

Efficiently Counting DISTINCT Values in Multiple Columns

In a scenario where you need to determine the count of unique combinations across multiple columns, a common approach is to utilize a subquery with the DISTINCT clause. However, this technique can sometimes be inefficient. Here, we explore a potential optimization method.

The provided query aims to count distinct "DocumentId" and "DocumentSessionId" combinations from the "DocumentOutputItems" table:

SELECT COUNT(*) 
FROM (SELECT DISTINCT DocumentId, DocumentSessionId
      FROM DocumentOutputItems) AS internalQuery

To enhance performance, consider generating a persisted computed column based on a hash or concatenated combination of the columns in question. Once created, this column can be indexed and have statistics associated with it, assuming suitable database settings.

By leveraging this persisted computed column, a DISTINCT count operation on the derived field should yield an equivalent result to the original query. This approach eliminates the need for a subquery and potentially improves efficiency. Here's an example:

ALTER TABLE DocumentOutputItems
ADD DocumentUniqueIndex AS HASHBYTES('MD5', DocumentId + DocumentSessionId) PERSISTED;

SELECT COUNT(DISTINCT DocumentUniqueIndex)
FROM DocumentOutputItems;

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