I'm about to implement a feature on my website that recommends content to users based on the content they already have in their library (a la Last.fm). A single table holds all the records of the content they have added, so a row might look something like:

```
--------------------
| userid | content |
--------------------
|   28   |    a    |
--------------------

```

When I want to recommend some content for a user, I use a query to get all the user id's that have content a added in their library. Then, out of those user id's, I make another query that finds the next most common content among those users (fx. 'b'), and show that to the user. 

My problem is when I'm thinking about the big picture here. Say that eventually my site will hold something like 500.000 rows in the table, will this make the MySQL response very slow or am I underestimating MySQL here?