Re: BitmapHeapScan streaming read user and prelim refactoring

On 14/03/2024 14:34, Robert Haas wrote:
> On Thu, Mar 14, 2024 at 6:37 AM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>> If es_snapshot was different from the active snapshot, things would get
>> weird, even without parallel query. The scans would use es_snapshot for
>> the visibility checks, but any functions you execute in quals would use
>> the active snapshot.
>
> Hmm, that's an interesting point.
>
> The case where the query is suspended and resumed - i.e. cursors are
> used - probably needs more analysis. In that case, perhaps there's
> more room for the snapshots to diverge.

The portal code is pretty explicit about it, the ExecutorRun() call in
PortalRunSelect() looks like this:

PushActiveSnapshot(queryDesc->snapshot);
ExecutorRun(queryDesc, direction, (uint64) count,
portal->run_once);
nprocessed = queryDesc->estate->es_processed;
PopActiveSnapshot();

I looked at all the callers of ExecutorRun(), and they all have the
active snapshot equal to queryDesc->snapshot, either because they called
CreateQueryDesc() with the active snapshot before ExecutorRun(), or they
set the active snapshot like above.

--
Heikki Linnakangas
Neon (https://neon.tech)