Re: ALTER TABLE lock strength reduction patch is unsafe

On 15 July 2013 15:06, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> Generally, the question on the table is: to what extent do MVCC
> catalog scans make the world safe for concurrent DDL, or put
> negatively, what hazards remain?

On Tom's test I've been unable to find a single problem.

> Noah discovered an interesting one recently: apparently, the relcache
> machinery has some logic in it that depends on the use of
> AccessExclusiveLock in subtle ways. I'm going to attempt to explain
> it, and maybe he can jump in and explain it better. Essentially, the
> problem is that when a relcache reload occurs, certain data structures
> (like the tuple descriptor, but there are others) are compared against
> the old version of the same data structure. If there are no changes,
> we do nothing; else, we free the old one and install the new one. The
> reason why we don't free the old one and install the new one
> unconditionally is because other parts of the backend might have
> pointers to the old data structure, so just replacing it all the time
> would result in crashes. Since DDL requires AccessExclusiveLock +
> CheckTableNotInUse(), any actual change to the data structure will
> happen when we haven't got any pointers anyway.
>
> A second thing I'm concerned about is that, although MVCC catalog
> scans guarantee that we won't miss a concurrently-updated row
> entirely, or see a duplicate, they don't guarantee that the rows we
> see during a scan of catalog A will be consistent with the rows we see
> during a scan of catalog B moments later. For example, hilarity will
> ensue if relnatts doesn't match what we see in pg_attribute. Now I
> don't think this particular example matters very much because I think
> there are probably lots of other things that would also break if we
> try to add a column without a full table lock, so we're probably
> doomed there anyway. But there might be other instances of this
> problem that are more subtle.

If you look at this as a generalised problem you probably can find
some issues, but that doesn't mean they apply in the specific cases
we're addressing.

The lock reductions we are discussing in all cases have nothing at all
to do with structure and only relate to various options. Except in the
case of constraints, though even there I see no issues as yet.

I've no problem waiting awhile to apply while others try to find loopholes.

--
Simon Riggs http://www.2ndQuadrant.com/
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