Assuming we're talking about the timeline "latest" mode (rather than "top"), IIRC in the past the used to do the home timeline processing on the read paths, but

Assuming we're talking about the timeline "latest" mode (rather than "top"), IIRC in the past the used to do the home timeline processing on the read paths, but have long since opted to optimize for read latency, thus the heavy lifting is performed during the insertion, so in broad strokes, when someone post a tweet, the three main things that happen are: 1. Insertion of tweet to tweets table. 2. Insertion of that tweet-id to the home timelines of all that user's followers. 3. Insertion of that tweet-id to the user-timeline of that user. On the read-path, if I'm not mistaken, the only join that happen is between the requested timeline and the tweets table (which is replicated across cluster of machines but not partitioned, or at least I remember reading that was the case not many years ago)