MySQL WHERE IF条件下的查询非常慢

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