Comment by amarant

17 hours ago

Thanks for writing this! The visualisations really drive a better understanding than pure text does, and it's quite clear that you have a better understanding of what database do under the hood than I do.

As such, I have a question for you: contrary to your article, I've always been taught that random primary keys are better than sequential ones. The reason for this, I was told, was to avoid "hotspots". I guess it only really applies once sharding comes into play, and perhaps also only if your primary key is your sharding key, but I think that's a pretty common setup.

I'm not really sure how to formulate a concrete question here, I guess I would like to hear your thoughts on any tradeoffs on sequential Vs random keys in sharded setups? Is there a case there random keys are valid, or have I been taught nonsense?

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amarant

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bddicken  17 hours ago

B+trees combined with sequential IDs are great for writes. This is because we are essentially just appending new rows to the "linked list" at the bottom level of the tree. We can also keep a high fill % if we know there isn't a lot of data churn.

If you're sharding based purely on sequential ID ranges, then yes this is a problem. Its better practice to shard based on a hash of your ID, so sequential id assignments turn into non-sequential shard keys, keeping things evenly distributed.

  • amarant  17 hours ago

    Oh wow, that's a super simple solution, and I can immediately see how this gets you the best of both worlds!

    And since it's only used for speedy lookup we can even use a fast, cheap and non-secure hashing algorithm, so it's really a low-cost operation!

    Thanks! This was really one of those aha-moments where I feel kinda stupid to not have thought of it myself!

  • cogman10  16 hours ago

    For our DBs (which are often unsharded), we've found the best performance using the user account ID as the first part of the cluster key and then a sequential id for whatever the record is as the second.

    It's not as good as just a sequential ID at keeping the fragmentation and data movement down. However, it does ultimately lead to the best write performance for us because the user data ends up likely still appending to an empty page. It allows for more concurrent writes to the same table because they aren't all fighting over that end page.

    UUIDv4 is madness.

traderj0e  16 hours ago

Spanner in particular wants random primary keys. But there are sharded DBMSes that still use sequential PKs, like Citus. There are also some use cases for semi-sequential PKs like uuid7.