If You have a purely theoretical question - then answer without specific implementations))) will Work relatively quickly if you regularly exercise protezirovanie or sharding (read on the Internet, a very useful thing even for a small DB). In short, is the separation of all the database partitions on which the muscle is much easier to work with than one large database. The muscle provides excellent tools for this. Moreover, he determines in which partition the desired data are stored at the time of the request. Also it does not impose absolutely no restrictions in working with joinme and indexes. The only negative is that it must be done manually. Although you can run a simple script on the crown, which would carry about a hundred requests protezirovanija only once a month. Load it almost does not create, but the muscle You will be very thankful that you relieve him of unnecessary work pulling the heavy bases. Sometimes with conjunction with it implemented sharding when automatically creates a new table in the database, when the old have accumulated a certain number of records (usually 10 000), with names table1, table2, table3, etc. In this case, different databases can generally be spread across different servers, but in most cases, due to some subjective factors, its implementation is usually not feasible, so in most cases widely used protezirovanie.
Also, as mentioned earlier - is unknown in what conditions it will use Your database: if requests to add more than reading - you need to use MyISAM, InnoDB otherwise, the difference is noticeable. Much.