Re: BitmapHeapScan streaming read user and prelim refactoring

On Wed, Mar 13, 2024 at 7:04 PM Heikki Linnakangas <hlinnaka(at)iki(dot)fi> wrote:
>
> (Adding Dilip, the original author of the parallel bitmap heap scan
> patch all those years ago, in case you remember anything about the
> snapshot stuff below.)
>
> On 27/02/2024 16:22, Melanie Plageman 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.

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