Multixacts (shared row locks)
Simple terms
A row has exactly one slot to record who is locking or deleting it. One transaction, one slot, no problem. Now a second transaction locks the same row for sharing. Two lockers, one slot. Postgres gets out of it by minting a separate id that stands for the whole group and storing that instead, so the slot now holds a pointer to a list rather than a transaction. That list lives in its own structure on disk, with its own wraparound clock to keep an eye on.
You might be asked
What is a multixact (MultiXactId), when does PostgreSQL create one, and how does it change the meaning of a tuple's t_xmax? Why do multixacts have their own wraparound horizon separate from transaction IDs?
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