Re: Optimize SnapBuildPurgeOlderTxn: use in-place compaction instead of temporary array

On Sat, 18 Oct 2025 at 12:50, Xuneng Zhou <xunengzhou(at)gmail(dot)com> wrote:
>
> Hi Hackers,

Hi!

> The SnapBuildPurgeOlderTxn function previously used a suboptimal
> method to remove old XIDs from the committed.xip array. It allocated a
> temporary workspace array, copied the surviving elements into it, and
> then copied them back, incurring unnecessary memory allocation and
> multiple data copies.
>
> This patch refactors the logic to use a standard two-pointer, in-place
> compaction algorithm. The new approach filters the array in a single
> pass with no extra memory allocation, improving both CPU and memory
> efficiency.
>
> No behavioral changes are expected. This resolves a TODO comment
> expecting a more efficient algorithm.
>

Indeed, these changes look correct.
I wonder why b89e151054a0 did this place this way, hope we do not miss
anything here.

Can we construct a microbenchmark here which will show some benefit?

--
Best regards,
Kirill Reshke