Currently, if old_snapshot_threshold is enabled, vacuum is prevented
from truncating tables:
static bool
should_attempt_truncation(LVRelStats *vacrelstats)
        BlockNumber possibly_freeable;

        possibly_freeable = vacrelstats->rel_pages - 
        if (possibly_freeable > 0 &&
                (possibly_freeable >= REL_TRUNCATE_MINIMUM ||
          possibly_freeable >= vacrelstats->rel_pages / REL_TRUNCATE_FRACTION) 
                old_snapshot_threshold < 0)
                return true;
                return false;

(note the old_snapshot_threshold < 0 condition).

That appears to not be mentioned in a comment, the commit message or the
the docs. I think this definitely needs to be prominently documented.

FWIW, afaics that's required because new pages don't have an LSN, so we
can't necessarily detect that a truncated and re-extended relation,
wouldn't be valid. Although I do wonder if there isn't a less invasive
way to do that.


Andres Freund

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