MySQL Query Tuning - Why is using a value from a variable so much slower than using a literal?

Question

UPDATE: I\'ve answered this myself below.

I\'m trying to fix a performance issue in a MySQL query. What I think I\'m seeing, is that assigning the r

Answer 1

Was finished writing the question when the answer hit me, so posting anyway for knowledge sharing!

I realised that the return value of the metaphone function was UTF8.

The compare to a latin1 field was obviously incurring a fairly heavy performance overhead.

I replaced the variable assignment with:

SET @metaphone_val:= CONVERT(double_metaphone(p_parameter) USING latin1);

Now the query runs as fast as I would expect.