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Right, I didn't mean to suggest that Postgres currently mmap(3)s all data; rather, I meant to get across that—in the scenario where you really want to keep all this data laying around as files rather than as LOBs—Postgres would need to have some way to guarantee that the data that's sitting around in those files is always in the state Postgres thinks it's in, if the corresponding file-streams are going to join in the same MVCC transactions as everything else. From what I can tell, that would require Postgres to do whatever it does for each table backing-file it keeps, to each of those files: both on the OS level (at least fopen(3) + flock(3)), and in terms of book-keeping in the system catalog.

Let me put it this way: how much overhead do you think there would be if you split your Postgres data across "ten billion" tablespaces? Because that's essentially what's being talked about here—little tablespaces containing one {oid, LOB} table, with one row, with a "storage engine" that represents that tablespace as a file.

...of course, if you do just (fix and then) use LOBs rather than insist on externally-visible files, none of that matters. :)

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