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> Postgres can’t update a JSON object on disk, it just rewrites the entire blob.

Postgres does this for _all_ row updates; it must, for transaction isolation to work.

I'm not sure, there's also HOT-updates (heap-only tuples). It's an optimization where data in modified in place if none of the modified columns are part of an index and maybe other conditions but I don't remember it all too well.

Did not know this; thank you!

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