In MySQL, which is more efficient: IFNULL or NULLIF?

Purpose of NULLIF and IFNULL is not same, so performance comparison does not make any sense.

NULLIF is used to return null if the expression has a specific value, whereas IFNULL is used to return text if expression is null.

Example:

SELECT IFNULL(field,'empty') from table1;

Since null does not make much sense to end user.

insert into table1 (field) values (nullif(field,'empty'));

Since null has a special meaning in database.

They are both as efficient as each other - the functions have about the same overhead as each other.

But this is more efficient than either:

(column_name is null 
 or column_name = 'test')

Because the function doesn't need to be invoked.

You may find putting the more commonly encountered test first improves performance.

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With questions like this, the simplest and more reliable way to discover relative performance is to just try them and compare query timings. Make sure you have production-sized and realistic-valued datasets so the test is fair.