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Reads (selects) won’t modify rows, but because of the internal bookkeeping that Postgres does, it can result in metadata that the WAL needs to write out to pages on disk. So in that sense, reads can result in writes.

This is such a relatable conclusion that echos many of my experiences chasing down odd production bugs:

Ops is like this a lot of the time. Once you get a working fix, you move on to whatever’s the next biggest source of anxiety. Sometimes you never get a fully satisfying “why.” But you can still love the chase.

Also, yes, Cybertec is always on top of it. One of the best in-depth Postgres blogs out there.

So I resort to Googling around for a while. I eventually land on this Cybertec blog post (there’s always a Cybertec post. God bless ’em), which demystifies shared buffers for me.