Easiest primary key for main table?

My main table, Users, stores information about users. I plan to have a UserId field as the primary key of the table. I have full control of creation and assignment of these keys, and I want to ensure that I assign keys in a way that provides good performance. What should I do?

You have a few options:

1) The most generic solution is to use UUIDs, as specified in RFC 4122.

For example, you could have a STRING(36) that stores UUIDs. Or you could store the UUID as a pair of INT64s or as a BYTE(16). There are some pitfalls to using UUIDs, so read the details of this answer.

2) If you want to save a bit of space and are absolutely sure that you will have fewer than a few billion users, then you could use an INT64 and then assign UserIds using a random number generator. The reason you want to be sure you have fewer than a few billion users is because of the Birthday Problem, the odds that you get at least one collision are about 50% once you have 4B users, and they increase very fast from there. If you assign a UserId that has already been assigned to a previous user, then your insertion transaction will fail, so you'll need to be prepared for that (by retrying the transaction after generating a new random number).

3) If there's some column, MyColumn, in the Users table that you would like to have as primary key (possibly because you know you'll want to look up entries using this column frequently), but you're not sure about the tendency of this column to cause hotspots (say, because it's generated sequentially or based on timestamps), then you two other options:

3a) You could "encrypt" MyColumn and use this as your primary key. In mathematical terms, you could use an automorphism on the key values, which has the effect of chaotically scrambling them while still never assigning the same value multiple times. In this case, you wouldn't need to store MyColumn separately at all, but rather you would only store/use the encrypted version and could decrypt it when necessary in your application code. Note that this encryption doesn't need to be secure but instead just needs to guarantee that the bits of the original value are sufficiently scrambled in a reversible way. For example: If your values of MyColumn are integers assigned sequentially, you could just reverse the bits of MyColumn to create a sufficiently scrambled primary key. If you have a more interesting use-case, you could use an encryption algorithm like XTEA.

3b) Have a compound primary key where the first part is a ShardId, computed ashash(MyColumn) % numShards and the second part is MyColumn. The hash function will ensure that you don't create a hot-spot by allocating your rows to a single split. More information on this approach can be found here. Note that you do not need to use a cryptographic hash, although md5 or sha512 are fine functions. SpookyHash is a good option too. Picking the right number of shards is an interesting question and can depend upon the number of nodes in your instance; it's effectively a trade-off between hotspot-avoiding power (more shards) and read/scan efficiency (fewer shards). If you only have 3 nodes, then 8 shards is probably fine. If you have 100 nodes; then 32 shards is a reasonable value to try.