ISNULL vs. COALESCE: Which SQL Function Offers Better Performance?

Using ISNULL vs COALESCE: Performance Implications and Best Practices

When dealing with conditional expressions and checking the existence of a specific field, developers often face the dilemma of choosing between COALESCE and ISNULL functions. Both functions return a value if the specified expression is NULL, but there are subtle differences and performance considerations that warrant further investigation.

Performance Comparison

One key difference between COALESCE and ISNULL lies in their performance impact. As highlighted in a report on Microsoft Connect, COALESCE can potentially lead to unnecessary subquery duplication under certain circumstances. This occurs when the expression being checked contains a subquery.

COALESCE Duplication:

COALESCE((SELECT Nullable FROM Demo WHERE SomeCol = 1), 1)

In this example, COALESCE rewrites the expression as:

CASE WHEN (SELECT Nullable FROM Demo WHERE SomeCol = 1) IS NOT NULL
THEN (SELECT Nullable FROM Demo WHERE SomeCol = 1)
ELSE 1
END

This leads to two executions of the subquery, potentially impacting performance, especially if the subquery is expensive.

ISNULL Avoidance:

ISNULL, on the other hand, does not suffer from the same subquery duplication issue. Instead, it rewrites the expression as:

CASE WHEN Nullable IS NOT NULL
THEN Nullable
ELSE 1
END

In this case, the subquery is executed only once, improving performance in situations similar to the example provided above.

Best Practice

For scenarios involving checking the existence of a single expression, ISNULL is generally the better option due to its performance advantage over COALESCE. It ensures that the subquery is executed only once, avoiding unnecessary duplication. However, COALESCE remains a valid choice when multiple parameters need to be checked or when the checked expression is more complex.

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