Why are we restricting exported snapshots in subtransactions?

On Mon, Jun 12, 2017 at 11:04 PM, Andres Freund <andres@anarazel.de> wrote:

ExportSnapshot() has, right at the beginning, the following block:

/*
* We cannot export a snapshot from a subtransaction because there's no
* easy way for importers to verify that the same subtransaction is still
* running.
*/
if (IsSubTransaction())
ereport(ERROR,
(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
errmsg("cannot export a snapshot from a subtransaction")));

that reasoning doesn't seem to make too much sense to me. Given that
exported snapshots don't make the exporting-transaction's changes
visible, I don't see why that restriction is needed?

As long as the exported snapshot enforces xmin to be retained, which it
does via the pairingheap, I don't understand why we'd have to enforce
that the subtransaction is still running?

I think you're touching on what is perhaps the key issue here, which
is that if it were possible to remove a tuple that the snapshot ought
to see before the snapshot got used, that would be bad. I don't
immediately see why we couldn't remove a tuple inserted by the aborted
subtransaction immediately. On a quick look, it seems to me that
HeapTupleSatisfiesVacuum() could fall through all the way to this
case:

/*
* Not in Progress, Not Committed, so either
Aborted or crashed
*/
SetHintBits(tuple, buffer, HEAP_XMIN_INVALID,
InvalidTransactionId);
return HEAPTUPLE_DEAD;

Even If I'm wrong about that, it seems like someone might want to make
me correct in the future - i.e. removing aborted tuples ASAP seems
like a good idea.

Another point to consider is whether a relfilenode assignment visible
to that snapshot might be a file that's since been truncated or
removed altogether.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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On 2017-06-13 13:15:57 -0400, Robert Haas wrote:

On Mon, Jun 12, 2017 at 11:04 PM, Andres Freund <andres@anarazel.de> wrote:

ExportSnapshot() has, right at the beginning, the following block:

/*
* We cannot export a snapshot from a subtransaction because there's no
* easy way for importers to verify that the same subtransaction is still
* running.
*/
if (IsSubTransaction())
ereport(ERROR,
(errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
errmsg("cannot export a snapshot from a subtransaction")));

that reasoning doesn't seem to make too much sense to me. Given that
exported snapshots don't make the exporting-transaction's changes
visible, I don't see why that restriction is needed?

As long as the exported snapshot enforces xmin to be retained, which it
does via the pairingheap, I don't understand why we'd have to enforce
that the subtransaction is still running?

I think you're touching on what is perhaps the key issue here, which
is that if it were possible to remove a tuple that the snapshot ought
to see before the snapshot got used, that would be bad.

I don't think that's really an issue here. Exported snapshots export a
state of the database of that moment, *except* that changes made by the
exporting transaction are not visible. IOW, a subtransaction's changes
aren't going to be visible anyway.

As far as I can tell that property makes:

I don't
immediately see why we couldn't remove a tuple inserted by the aborted
subtransaction immediately. On a quick look, it seems to me that
HeapTupleSatisfiesVacuum() could fall through all the way to this
case:

Even If I'm wrong about that, it seems like someone might want to make
me correct in the future - i.e. removing aborted tuples ASAP seems
like a good idea.

Another point to consider is whether a relfilenode assignment visible
to that snapshot might be a file that's since been truncated or
removed altogether.

All pretty much moot?

The reason subxids are exported right now is to *ignore* changes made by
them. But for that we could just as well set suboverflowed to true, and
just include the toplevel xid in ->xip[].

- Andres

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