Re: WIP patch for parallel pg_dump

Tom Lane <tgl@sss.pgh.pa.us> wrote:
> "Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
>> Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> No.  See subtransactions.
> 
>> Subtransactions are included in snapshots?
> 
> Sure, see GetSnapshotData().  You could avoid it by setting
> suboverflowed, but that comes at a nontrivial performance cost.
 
Yeah, sorry for blurting like that before I checked.  I was somewhat
panicked that I'd missed something important for SSI, because my
XidIsConcurrent check just uses xmin, xmax, and xip; I was afraid
what I have would fall down in the face of subtransactions.  But on
review I found that I'd thought that through and (discussion in in
the archives) I always wanted to associate the locks and conflicts
with the top level transaction; so that was already identified
before checking for overlap, and it was therefore more efficient to
just check that.
 
Sorry for the "senior moment".  :-/
 
Perhaps a line or two of comments about that in the SSI patch would
be a good idea.  And maybe some tests involving subtransactions....
 
-Kevin