Is calling a function multiple times in a MySQL query optimized?

Question

MySQL

In this query, will the myfun() user-defined function be evaluated three times for each field value, or will there be some kind of optimization? What can be done to optimize such a query?

SELECT COUNT(*) FROM mytable 
WHERE myfun( field )='str1' OR myfun( field )='str2' OR myfun( field )='str3';

Answer 1

SELECT COUNT(*) FROM mytable 
WHERE myfun( field ) IN ('str1', 'str2', 'str3');

but it's still bad. It's better to find an inverse function and use

SELECT COUNT(*) FROM mytable 
WHERE field IN (myrevfunc('str1'), myrevfunc('str2'), myrevfunc('str2'));

Answer 2

will be calculated. and everything will be past the indexes.
index over generated column
with some restrictions on the implementation of myfun ()
if it cannot be expressed through allowed operators / literals / function calls, then you have to pervert triggers to calculate the values ​​of the indexed field.