On Tue, Jun 14, 2016 at 8:08 AM, Robert Haas <robertmh...@gmail.com> wrote:
> On Tue, Jun 14, 2016 at 2:53 AM, Thomas Munro
> <thomas.mu...@enterprisedb.com> wrote:
>> On Tue, Jun 14, 2016 at 4:02 AM, Robert Haas <robertmh...@gmail.com> wrote:
>>> On Sat, Jun 11, 2016 at 5:00 PM, Robert Haas <robertmh...@gmail.com> wrote:
>>>>> How about changing the return tuple of heap_prepare_freeze_tuple to
>>>>> a bitmap?  Two flags: "Freeze [not] done" and "[No] more freezing
>>>>> needed"
>>>> Yes, I think something like that sounds about right.
>>> Here's a patch.  I took the approach of adding a separate bool out
>>> parameter instead.  I am also attaching an update of the
>>> check-visibility patch which responds to assorted review comments and
>>> adjusting it for the problems found on Friday which could otherwise
>>> lead to false positives.  I'm still getting occasional TIDs from the
>>> pg_check_visible() function during pgbench runs, though, so evidently
>>> not all is well with the world.
>> I'm still working out how half this stuff works, but I managed to get
>> pg_check_visible() to spit out a row every few seconds with the
>> following brute force approach:
>> CREATE TABLE foo (n int);
>> INSERT INTO foo SELECT generate_series(1, 100000);
>> Three client threads (see attached script):
>> 1.  Run VACUUM in a tight loop.
>> 2.  Run UPDATE foo SET n = n + 1 in a tight loop.
>> 3.  Run SELECT pg_check_visible('foo'::regclass) in a tight loop, and
>> print out any rows it produces.
>> I noticed that the tuples that it reported were always offset 1 in a
>> page, and that the page always had a maxoff over a couple of hundred,
>> and that we called record_corrupt_item because VM_ALL_VISIBLE returned
>> true but HeapTupleSatisfiesVacuum on the first tuple returned
>> It did that because HEAP_XMAX_COMMITTED was not set and
>> TransactionIdIsInProgress returned true for xmax.
> So this seems like it might be a visibility map bug rather than a bug
> in the test code, but I'm not completely sure of that.  How was it
> legitimate to mark the page as all-visible if a tuple on the page
> still had a live xmax?  If xmax is live and not just a locker then the
> tuple is not visible to the transaction that wrote xmax, at least.

Ah, wait a minute.  I see how this could happen.  Hang on, let me
update the pg_visibility patch.

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

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