On Wed, Jul 13, 2016 at 12:48 PM, Andres Freund <and...@anarazel.de> wrote: > On 2016-07-13 10:06:52 -0500, Kevin Grittner wrote: >> On Wed, Jul 13, 2016 at 7:57 AM, Amit Kapila <amit.kapil...@gmail.com> wrote: >>> On Tue, Jul 12, 2016 at 8:34 PM, Kevin Grittner <kgri...@gmail.com> wrote: >>>> On Fri, Jul 8, 2016 at 1:52 PM, Andres Freund <and...@anarazel.de> wrote: >>>> >>>>> I'm a bit confused, why aren't we simply adding LSN interlock >>>>> checks for toast? Doesn't look that hard? Seems like a much more >>>>> natural course of fixing this issue? >>>> >>>> I took some time trying to see what you have in mind, and I'm >>>> really not "getting it". >>> >>> Isn't it possible if we initialize lsn and whenTaken in SnapshotToast >>> when old_snapshot_threshold > 0 and add a check for >>> HeapTupleSatisfiesToast in TestForOldSnapshot()? >> >> With that approach, how will we know *not* to generate an error >> when reading the chain of tuples for a value we are deleting. Or >> positioning to modify an index on toast data. Etc., etc. etc. > > I'm not following. How is that different in the toast case than in the > heap case?
A short answer is that a normal table's heap doesn't go through systable_getnext_ordered(). That function is used both for cases where the check should not be made, like toast_delete_datum(), and cases where it should, like toast_fetch_datum(). Since this keeps coming up, I'll produce a patch this way. I'm skeptical, but maybe it will look better than I think it will. I should be able to post that by Friday. -- Kevin Grittner EDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
