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

On Wed, Jun 15, 2016 at 1:44 PM, Kevin Grittner <kgrittn(at)gmail(dot)com>
wrote:>> So what happens in this scenario:
>> 1. ANALYZE runs really slowly - maybe the user-defined function it's
>> running for the expression index is extremely long-running.
>> 2. Eventually, the snapshot for ANALYZE is older than the configured
>> value of snapshot_too_old.
>> 3. Then, ANALYZE selects a page with an LSN new enough that it might
>> have been pruned.
>>
>> Presumably, the ANALYZE ought to error out in this scenario, just as
>> it would in any other situation where an old snapshot sees a new page.
>> No?
>
> 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.

Maybe I'm confused.

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