Re: unnecessary executor overheads around seqscans

Hi,

On 2026-01-24 15:23:44 +0530, Amit Kapila wrote:
> On Sat, Jan 24, 2026 at 1:46 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> >
> > - The checkXidAlive checks that have been added to table_scan_getnextslot()
> > show up noticeably and in every loop iteration, despite afaict never being reachable
> >
> > It's not obvious to me that this should
> > a) be in table_scan_getnextslot(), rather than in beginscan - how could it
> > change in the middle of a scan? That would require a wrapper around
> > rd_tableam->scan_begin(), but that seems like it might be good anyway.
> > b) not just be an assertion?
> >
>
> IIRC, the main reason for having this precautionary check in the API
> is to ensure that during logical decoding we never access the table AM
> or
> heap APIs directly when scanning catalog tables. This restriction
> exists because we only check for concurrent aborts inside the
> systable_* APIs.

I know why the check exists - but why does it have to be in
table_scan_getnextslot(), which is executed very frequently, rather than
table_beginscan*(), which is executed much less frequently.

Greetings,

Andres Freund