Re: Usage of epoch in txid_current

On Fri, Jun 21, 2019 at 7:01 PM Alexander Korotkov
<a(dot)korotkov(at)postgrespro(dot)ru> wrote:
> On Thu, Mar 28, 2019 at 8:30 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> > Thanks for the reviews! Pushed.
>
> Any ideas we should move towards 64-bit xids in more places? That has
> been discussed several times already. I think last time it was
> discussed in person during FOSDEM PGDay 2018 Developer Meeting [1].
> There we've discussed that it probably doesn't worth it to change
> 32-bit on-disk xids in heap. It's better to leave existing heap "as
> is", but allow other pluggable table access methods to support 64-bit
> xids. Given now we have pluggable table access methods, we may build
> a plan on full support of 64-bit xids in core.
>
> In my vision sketchy plan may look like this.
>
> 1. Change all non-heap types of xids from TransactionId to
> FullTransactionId.

I think it's fine to replace TransactionId with FullTransactionId
without stressing about it too much in code that's not that heavily
trafficked. However, I'm not sure we can do that across the board. For
example, converting snapshots to use 64-bit XIDs would mean that in
the worst case a snapshot will use twice as many cache lines, and that
might have performance implications on some workloads.

--
Robert Haas
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