Re: Heap truncation without AccessExclusiveLock (9.4)

On Thu, May 16, 2013 at 1:15 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>>> 2. If you don't find an entry for your target rel in the cache, aren't
>>> you still going to have to do an lseek?
>
>> Don't think of it as a cache.  The caching happens inside each
>> backend's relcache; the shared memory structure is just a tool to
>> force those caches to be revalidated when necessary.
>
> Hmm.  Now I see: it's not a cache, it's a Bloom filter.

Yes.

> The failure
> mode I was thinking of is inapplicable, but there's a different one:
> you have to be absolutely positive that *any* operation that extends the
> file will update the relevant filter entry.  Still, I guess that we're
> already assuming that any such op will take the relation's extension
> lock, so it should be easy enough to find all the places to fix.

I would think so.  The main thing that's held me back from actually
implementing this is the fact that lseek is so darn cheap on Linux,
and I don't have reliable data one way or the other for any other
platform.

-- 
Robert Haas
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