Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

On Fri, May 27, 2016 at 10:18 AM, Kevin Grittner <kgrittn(at)gmail(dot)com> wrote:
> On Fri, May 27, 2016 at 9:58 AM, Kevin Grittner <kgrittn(at)gmail(dot)com> wrote:
>> On Tue, May 24, 2016 at 4:56 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
>>> If an old session with >= repeatable read accesses a clustered
>>> table (after the cluster committed), they'll now not see any
>>> errors, because all the LSNs look new.
>>
>> Again, it is new LSNs that trigger errors; if the page has not been
>> written recently the LSN is old and there is no error. I think you
>> may be seeing problems based on getting the basics of this
>> backwards.
>
> I am reviewing the suggestion of a possible bug now, and will make
> it my top priority until resolved. By the end of 1 June I will
> either have committed a fix or posted an explanation of why the
> concern is mistaken, with test results to demonstrate correct
> behavior.

This got set back by needing to fix a bug in the 9.5 release. I am
back on this and have figured out that everyone who commented on
this specific issue was wrong about a very important fact -- the
LSNs in index pages after CREATE INDEX (with or without
CONCURRENTLY) and for REINDEX are always == InvalidXLogRecPtr (0).

That means that a snapshot from before an index build does not
always generate errors when it should on the use of the new index.
(Any early pruning/vacuuuming from before the index build is
missed; activity subsequent to the index build is recognized.)
Consequently, causing the index to be ignored in planning when
using the old index is not a nice optimization, but necessary for
correctness. We already have logic to do this for other cases
(like HOT updates), so it is a matter of tying in to that existing
code correctly. This won't be all that novel.

I now expect to push a fix along those lines by Tuesday, 6 June.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company