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 result of a function to a variable, and then running a SELECT with a compare against that variable is relatively slow.

If for testings sake however, I replace the compare to the variable with a compare to the string literal equivalent of what I know that function will return (for a given scenario), then the query runs much faster.

For example:

...

SET @metaphone_val := double_metaphone(p_parameter)); -- double metaphone is user defined

SELECT 

        SQL_CALC_FOUND_ROWS
        t.col1,
        t.col2, 
        ...

    FROM table t

            WHERE

            t.pre_set_metaphone_string = @metaphone_val -- OPTION A

            t.pre_set_metaphone_string = 'PRN' -- OPTION B (Literal function return value for a given name)

If I use the line in option A, the query is slow.

If I use the line in option B, then the query is fast as you would expect any simple string compare to be.

Why?

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.