IFNULL in mysql speed issue

Question

select IFNULL(col1, (select col2 from table2 where ...)) from table1

Will it run select for table2 if table1 would have not null value? This is a speed issue question. I have no appropriate database to check.

I'm just designing database. I do not have a stand with heavy tables to test. — Yevgeniy Afanasyev, Mar 14 '13 at 05:30
can you make col2 in table2 primary key? because the database will have indexes for primary keys which will improve performance — Bhavik Shah, Mar 14 '13 at 05:46
I don not need to improve performance. I want to know how it works. I can not check it because I have no local database big enough to have significant differences to come to conclusion. — Yevgeniy Afanasyev, Mar 14 '13 at 06:01

score 0 · Accepted Answer · edited Jan 11 '17 at 02:12

0

The sub-query will only be executed if col1 is null but each time it is null. So if there are many null values in col1 this will become very slow.

The following may work better for you:

select coalesce(col1,col2) from table1,table2 
                     where (relationship between both tables)

edited Jan 11 '17 at 02:12

ρяσѕρєя K

answered Jan 11 '17 at 01:45

Mr Bear

1 Answers1