I don't want to start a discussion so I am keeping my question to numbers.
I have a table in my MySQL database that has 3099916 records. Now two columns of the table are the same except for a single character.
In my case they are URL and Thumbnail_URL from the same image.
Size of URL string is 72 characters.
like image URL is 12345sss_a.jpg
while Thumbnail URL is 12345sss_b.jpg
Now what'd be better for me, i.e the most efficient way
Depends on size of your data base.
Under current circumstances replacing is much much better, because your table is pretty much big.
You can only store the uploaded image name into the database and upload the real image and also thumb images to server and give their name like imagename_80x80.jpg, imagename_200x200.jpg.
When you want to show any image, just get the real name of the image from the database and put a postfix image size that size's thumbnail you will want.
In this way, you can upload multiple thumbnails and improve your query performance.
Plus one for the benchmarking.
But actually, I doubt you'll be able to tell the difference unless you have a resource constraint somewhere.
This is all based on assumptions, but I guess you have PHP code that retrieves the image information based on some criteria. The expensive part in this is the "where" clause - if that's slow, everything else will be slow.
The number of columns you return from this query makes no noticable difference on performance, unless you are returning a HUGE number of columns, or the columns are very large.
So, you may as well retrieve both image and thumbnail columns in the same query.
On the other hand - the string replace function in PHP is pretty fast too, so again - unless you're showing a LOT of images on the page, you're unlikely to see a measurable difference. And if you're showing a lot of images on the page, download time and browser rendering is much more of an issue than the PHP string replace...
Store only 12345sss. When SELECTing, tack on the suffix to get the image or the thumb.