> I haven't been paying much attention, but I imagine that what's really
> going on here is that once vacuum has collected all the still-good
> tuples at the front of the relation, it doesn't bother to go through
> the remaining blocks of the relation and mark everything dead therein?
> It just truncates the file after the last block that it put tuples into,
> right?
>
> If this procedure works correctly for vacuuming a simple one-segment
> table, then it would seem that truncation of all the later segments to
> zero length should work correctly.
Not sure about that. When we truncate single segment file, the table is
being destroyed, so we invalidate it in the catalog cache and tell other
backends. Also, we have a problem with DROP TABLE in a transaction
while others are using it as described by a bug report a few days ago,
so I don't think we have that 100% either.
> You could truncate to zero length *and* then unlink the files if you
> had a mind to do that, but I can see why unlink without truncate would
> not work reliably.
That is interesting. I never thought of that. Hiroshi, can you test
that idea? If it is the non-existance of the file that other backends
are checking for, my earlier idea of rename() with truncated file kept
in place may be better.
Also, I see why we are not getting more bug reports. They only get this
when the table looses a segment, so it is OK to vacuum large tables as
long as the table doesn't loose a segment during the vacuum.
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