Re: GetCurrentVirtualXIDs()

Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> No need to wait for idle-in-transaction sessions during index builds.
> GetCurrentVirtualXIDs() specifically *includes* backends that have
> proc->xmin == InvalidTransactionId (0), but I'm not sure why.

On further consideration, this patch is simply *wrong*, and would still
be wrong even if we changed GetCurrentVirtualXIDs to take ProcArrayLock
exclusive instead of shared.

If a backend currently has no snapshot, then if it takes a snapshot
immediately after we finish running GetCurrentVirtualXIDs, it will
set its proc->xmin (and that of the snapshot) to the oldest currently
running XID. There is no reason to assume that that value is >=
limitXmin, which is what you propose we do.

A safe modification of the patch would be to determine the oldest
running XID and exclude xmin-less VXIDs when that number is greater
than limitXmin. However, I think that that would be pretty useless:
for the one current use of GetCurrentVirtualXIDs, limitXmin is the
xmax of the snapshot we used for the index build, and we can assume
that our *own* XID is less than that, never mind anyone else's.

So I don't think this works...

regards, tom lane