Question

I am using the following MySQL query which is working fine, I mean giving me the desired output but… lets first see the query:

select 
    fl.file_ID,
    length(fl.filedesc) as l,
    case 
        when 
            fl.part_no is null
            and l>60
            then concat(fl.fileno,' ', left(fl.filedesc, 60),'...')
        when
            fl.part_no is null
            and length(fl.filedesc)<=60
            then concat(fl.fileno,' ',fl.filedesc)
        when
            fl.part_no is not null
            and length(fl.filedesc)>60
            then concat(fl.fileno,'(',fl.part_no,')', left(fl.filedesc, 60),'...')      
        when
            fl.part_no is not null
            and length(fl.filedesc)<=60
            then concat(fl.fileno,'(',fl.part_no,')',fl.filedesc)
        end as filedesc
from filelist fl

I don’t want to use the length function repeatedly because I guess it would hit the database everytime claiming performance issue. Please suggest if I can store the length once and use it several times.

Answer 1

Once you have accessed a given row, what you do with the columns has only a small impact on performance. So your guess that it “hits the database” more to serve repeated use of that length function isn’t as bad as you think.

The analogy I would use is a postal carrier delivering mail to your house, which is miles outside of town. He drives for 20 minutes to your mailbox, and then he worries that it takes too much time to insert one letter at a time into your mailbox, instead of all the letters at once. The cost of that inefficiency is insignificant compared to the long drive.

That said, you can make the query more concise or easier to code or to look at. But this probably won’t have a big benefit for performance.

select 
    fl.file_ID,
    concat(fl.fileno,
           ifnull(concat('(',fl.part_no,')'), ' '),
           left(fl.filedesc,60),
           if(length(fl.filedesc)>60,'...','')
    ) as filedesc
from filelist fl