Hi Heikki,

On Tue, Apr 10, 2018 at 7:07 PM, Heikki Linnakangas <hlinn...@iki.fi> wrote:

> It would seem more straightforward to add a snapshot parameter to
> GetNewOidWithIndex(), so that the this one caller could pass SnapshotToast,
> while others pass SnapshotDirty. With your patch, you check the index
> twice: first with SnapshotDirty, in GetNewOidWithIndex(), and then with
> SnapshotToast, in the caller.

Hmm. I actually wrote my first patch exactly like that. I am trying to
remember why I discarded that approach. Was that to do with the fact
that  SnapshotToast
can't see all toast tuples either? Yeah, I think so. For example, it won't
see tuples with uncommitted-xmin, leading to different issues. Now it's
unlikely that we will have a OID conflict where the old tuple has
uncommitted-xmin, but not sure if we can completely rule that out. For
example, if we did not uncover the redo recovery bug, we could have had a
prepared transaction having inserted the old tuple, which now conflicts
with new tuple and not detected by SnapshotToast.

> If I'm reading the rewrite case correctly, it's a bit different and quite
> special. In the loop with GetNewOidWithIndex(), it needs to check that the
> OID is unused in two tables, the old TOAST table, and the new one. You can
> only pass one index to GetNewOidWithIndex(), so it needs to check the
> second index manually. It's not because of the snapshot issue. Although I
> wonder if we should be using SnapshotToast in that GetNewOidWithIndex()
> call, too. I.e. if we should be checking both the old and the new toast
> table with SnapshotToast.

As I said, I am not sure if checking with just SnapshotToast is enough
because it can't see "dirty" tuples.

> Agreed. With nextXid, we advance ShmemVariableCache->nextXid if the value
> in the online checkpoint record is greater than
> ShmemVariableCache->nextXid. But we don't have such a wraparound-aware
> concept of "greater than" for OIDs. Ignoring the online checkpoint record's
> nextOid value seem fine to me.

Ok. Thanks for checking.


 Pavan Deolasee                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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