* Claudio Freire (klaussfre...@gmail.com) wrote: > On Thu, Oct 2, 2014 at 6:19 PM, Stephen Frost <sfr...@snowman.net> wrote: > >> And that needs locks, especially if you modify the underlying filesystem > >> layout. > > > > And we wouldn't be doing that, certainly. It's a dry-run. > > ... > > > (...) We might also be able to simply get away with > > short-circuiting them and not actually writing anything (and the same > > for reading data..). What would probably be useful is to review actual > > migration scripts and see if this would really work. I know they'd work > > for at least a subset of the migration scripts that I've dealt with > > before, and also not all of them. > > I believe, for it to be reasonably intrusive, you'd have to abort at > the first need to actually read/write data. > > Most of my migration scripts, especially the ones that would benefit > from this, require some of that, but that's just my personal common > practice, not a general case.
I realize we're moving into real pie-in-the-sky ideas here, but I wonder
if we could have a way to either create a new 'delta' fork which
maintains the pages which have been changed.. Or perhaps actually have
a new relfileno with a reference back to the original for any empty
pages..
> > The point is to not write anything which is visible to other
> > transactions, which means we'd have to put DML into some different
> > 'mode' which doesn't actually write where other processes might be
> > looking. I'm not saying it's trivial to do, but I don't think it's
> > impossible either. (...)
>
> No, I don't think it's impossible either. Just very, very
> time-consuming. Both in developing the patch and in its eventual
> maintenance.
I agree that we don't want a completely independent code path for this
or it won't be as useful- and that's a concern for the EXPLAIN idea too.
> TBH, a separate read-only transaction like explain alter would also be
> quite difficult to keep in sync with actual alter logic, unless it's
> handled by the same code (unlikely in that form).
Right.
Thanks,
Stephen
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