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This can easily happen if you have multiple services that can write/update in the same table/rows and rely on database coordination instead of using an external queue like Kafka.

Postgres (presumably other databases) can propagate these locks to table locks [1] and cause contention for the whole infra.

[1] https://blog.heroku.com/curious-case-table-locking-update-qu...

SELECT FOR UPDATE means two queries can’t _read_ the same row at the same time, right?

To my understanding, yes. If you have broad select queries across all the rows, that would slow things down quite a bit.

You can use SKIP LOCKED normally. That’s how popular queue implementations usually work. I believe pg even calls this out in their docs

SQL Server will escalate locks. Because keeping a million rows locked is expensive.