NVL vs. COALESCE: When Does Efficiency Trump Familiarity?

Deep dive into the differences between NVL and COALESCE

The main difference between NVL and COALESCE is the number of parameters and evaluation behavior, but there are some subtle differences to note. Unlike NVL, COALESCE is an ANSI-92 standard function, while NVL is an Oracle proprietary function. However, for the two-argument case, the two functions behave exactly the same.

The key difference is how they are implemented. NVL evaluates all parameters regardless of whether they are empty or not. In comparison, COALESCE is more efficient because it stops evaluation as soon as it encounters the first non-null value.

This difference has practical implications. Consider the following example:

<code class="language-sql">SELECT SUM(val)
FROM (
  SELECT NVL(1, LENGTH(RAWTOHEX(SYS_GUID()))) AS val
  FROM dual
  CONNECT BY level < 10000
)</code>

NVL mercilessly generates SYS_GUIDs for all 10000 iterations, resulting in query execution times approaching half a second.

Now, let’s compare COALESCE:

<code class="language-sql">SELECT SUM(val)
FROM (
  SELECT COALESCE(1, LENGTH(RAWTOHEX(SYS_GUID()))) AS val
  FROM dual
  CONNECT BY level < 10000
)</code>

COALESCE recognizes that 1 is a non-null value and stops evaluating the second argument. This eliminates unnecessary SYS_GUID generation and makes queries almost instantaneous.

In summary, while NVL and COALESCE look similar when dealing with two values, their different implementations suggest that COALESCE provides greater efficiency by selectively evaluating parameters based on null status. This optimization can significantly improve query performance, especially in scenarios involving complex and iterative evaluations.

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