MySQL very slow query with WHERE IF condition

Question

Referring below queries. The 1st query runs very slow even though the logic is same like the 2nd query. Tables table01 and table02 have about ~15000 records each. So what is exactly the problem here? I already exhausted of idea.

-- 1st. Very slow
hardcoded here for easier reference
SELECT a.eventid, d.fileno
FROM table01 a LEFT JOIN table02 d ON a.eventid=d.eventid
WHERE IF('fileno'='fileno',d.fileno = 'FIL123',0=0)
-- original: WHERE IF(search_field='fileno',d.fileno = 'BC12449',0=0)
-- search_field is passsed from stored procedure

-- 2nd. Fast
SELECT a.eventid, d.fileno
FROM table01 a LEFT JOIN table02 d ON a.eventid=d.eventid
WHERE d.fileno = 'FIL123'

Answer 1

Any indexes you may have on columns which names you generate inside the if clause will not be able to be used in the first example.

In general, wraping a column name inside any function with mysql will negate indexing benifits on the column