I will write this question using a database example, but please note this detail may be irrelevant.
Let's say I have data belonging to N clients stored in a single column of a database table. Each client has their own AES key used to encrypt/decrypt their own data.
In this setting, I have the understanding that if someone gets a copy of all values in the database, then brute-force attacks would actually become easier than if I had used just one single key to encrypt the entire column instead of this one-key-per-client approach.
If I had used just a single key for the entire data, the attacker would need to find that one key that would correctly decrypt all of the data. However, if one different key has been used to encrypt each of those N partitions, then time for finding at least one key that would decrypt at least one of these partitions would be reduced by a factor of N, since each candidate key being tested during a brute-force attack could match any of the N keys instead of having to match just one.
In this situation, it seems all that I have done was to reduce the search surface for finding a valid key from X to X/N.
Edited for clarification - All the above mentioned, my question is: Is having a single column with data encrypted according to different keys could possibly make brute-forcing the data easier?
If yes, what would be a suitable approach to overcome this, i.e., for encrypting different groups of data using separate keys without reducing the difficulty of brute-forcing individual keys?