Re: Improvement of procArray.xmin for VACUUM

Bruce Momjian <bruce(at)momjian(dot)us> writes:
> Tom Lane wrote:
>> ... but we won't come out ahead unless advancing xmin
>> intra-transaction really helps, and I'm not sure I believe that (except
>> in the special case of VACUUM, and we already have a solution for that,
>> which would be independent of this).

> The improvement is going to be a win for multi-statement transactions
> --- the only question is how often are they long-running.

Uh, no, that's not very clear. A long-running transaction will be a
VACUUM bottleneck because of its own XID, never mind its xmin. To make
this helpful, you have to posit a lot of overlapping long-running
transactions (such that the distance back to GlobalXmin might average
about twice the distance back to the oldest live XID). That's not
impossible but I wonder whether it's not mostly a token of bad
application design.

> It does seem best to put this on the TODO for 8.4, and I will do that
> now.

Agreed. Quite aside from the time needed for a reasonable
implementation, we'd really need to do more performance-testing than we
have time for now.

regards, tom lane