问题
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?
回答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.
来源：https://stackoverflow.com/questions/14986977/mysql-query-tuning-why-is-using-a-value-from-a-variable-so-much-slower-than-us