Re: BitmapHeapScan streaming read user and prelim refactoring

On Wed, Mar 13, 2024 at 9:25 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Wed, Mar 13, 2024 at 11:39 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> > > Andres already commented on the snapshot stuff on an earlier patch
> > > version, and that's much nicer with this version. However, I don't
> > > understand why a parallel bitmap heap scan needs to do anything at all
> > > with the snapshot, even before these patches. The parallel worker
> > > infrastructure already passes the active snapshot from the leader to the
> > > parallel worker. Why does bitmap heap scan code need to do that too?
> >
> > Yeah thinking on this now it seems you are right that the parallel
> > infrastructure is already passing the active snapshot so why do we
> > need it again. Then I checked other low scan nodes like indexscan and
> > seqscan and it seems we are doing the same things there as well.
> > Check for SerializeSnapshot() in table_parallelscan_initialize() and
> > index_parallelscan_initialize() which are being called from
> > ExecSeqScanInitializeDSM() and ExecIndexScanInitializeDSM()
> > respectively.
>
> I remember thinking about this when I was writing very early parallel
> query code. It seemed to me that there must be some reason why the
> EState has a snapshot, as opposed to just using the active snapshot,
> and so I took care to propagate that snapshot, which is used for the
> leader's scans, to the worker scans also. Now, if the EState doesn't
> need to contain a snapshot, then all of that mechanism is unnecessary,
> but I don't see how it can be right for the leader to do
> table_beginscan() using estate->es_snapshot and the worker to use the
> active snapshot.

Yeah, that's a very valid point. So I think now Heikki/Melanie might
have got an answer to their question, about the thought process behind
serializing the snapshot for each scan node. And the same thing is
followed for BitmapHeapNode as well.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com