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

On Wed, Jun 15, 2016 at 2:45 PM, Alvaro Herrera
<alvherre@2ndquadrant.com> wrote:
> Robert Haas wrote:
>> On Wed, Jun 15, 2016 at 1:44 PM, Kevin Grittner <kgrittn@gmail.com>
>> wrote:
>> > The test I showed creates a situation which (to ANALYZE) is
>> > identical to what you describe -- ANALYZE sees a page with an LSN
>> > recent enough that it could have been (and actually has been)
>> > pruned.  Why would it be better for the ANALYZE to fail than to
>> > complete?
>>
>> As I understand it, the reason we need to sometimes give "ERROR:
>> snapshot too old" after early pruning is because we might otherwise
>> give the wrong answer.
>
> So what constitutes "the wrong answer"?  A regular transaction reading a
> page and not finding a tuple that should have been there but was
> removed, is a serious problem and should be aborted.  For ANALYZE it may
> not matter a great deal.  Some very old tuple that might have been
> chosen for the sample is not there; a different tuple is chosen instead,
> so the stats might be slightly difference.  No big deal.
>
> Maybe it is possible to get into trouble if you're taking a sample for
> an expression index.

The expression index case is the one to worry about; if there is a
problem, that's where it is.  What bothers me is that a function used
in an expression index could do anything at all - it can read any
table in the database.

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