Question

I am making a website with a large pool of images added by users.

I want to choose randomly one image out of this pool, and display it to the user, but I want to make sure that this user has never seen this image before.

So i was thinking that: when a user views an image, I make a row INSERT in MYSQL that would say “This USER has watched THIS IMAGE at (TIME)” for every entry.
But the thing is, since there might be a lot of users and a lot of images, this table can easily grow to tens of thousands of entries quite rapidly.

So alternatively, it might be done like that:
I was thinking of making a row INSERT for every USER, and in ONE field, I insert an array all id’s of images that user has watched.

I can even do that to the array:
base64_encode(gzcompress(serialize($array)

And then:
unserialize(gzuncompress(base64_decode($array))

What do you think I should do?
Is the encoding/decoding functions fast enough, or at least faster than the conventional way i was describing at the beginning of the post?

Is that compression good enough to store large chunks of data into only ONE database field? (imagine if the user has viewed thousands images?)

Thanks a lot

Answer 1

in ONE field, I insert an array all id’s

In almost all cases, serializing values like this is bad practice. Let the database do what it’s designed to do — efficiently handle large amounts of data. As long as you ensure that your cross table has an index on the user field, retrieving the list of images that a user has seen will not be an expensive operation, regardless of the number of rows in the table. Tens of thousands of entries is nothing.