Re: Parallel Index Scans

On Thu, Jan 19, 2017 at 1:18 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:

> On Wed, Jan 18, 2017 at 6:25 AM, Haribabu Kommi
> <kommi(dot)haribabu(at)gmail(dot)com> wrote:
> >
> >
> > On Mon, Jan 16, 2017 at 11:11 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> > wrote:
> >>
> >
> > + * index_beginscan_parallel - join parallel index scan
> >
> > The name and the description doesn't sync properly, any better
> description?
> >
>
> This can be called by both the worker and leader of parallel index
> scan. What problem do you see with it. heap_beginscan_parallel has
> similar description, so not sure changing here alone makes sense.
>

Ok.

>
> >
> > +extern BlockNumber _bt_parallel_seize(IndexScanDesc scan, bool
> *status);
> > +extern void _bt_parallel_release(IndexScanDesc scan, BlockNumber
> > scan_page);
> >
> > Any better names for the above functions, as these function will
> provide/set
> > the next page that needs to be read.
> >
>
> These functions also set the state of scan. IIRC, these names were
> being agreed between Robert and Rahila as well (suggested offlist by
> Robert). I am open to change if you or others have any better
> suggestions.
>

I didn't find any better names other than the following,

_bt_get_next_parallel_page
_bt_set_next_parallel_page

> >
> > + /* reset (parallel) index scan */
> > + if (node->iss_ScanDesc)
> > + {
> >
> > Why this if check required? There is an assert check in later function
> > calls.
> >
>
> This is required because we don't initialize the scan descriptor for
> parallel-aware nodes during ExecInitIndexScan. It got initialized
> later at the time of execution when we initialize dsm. Now, it is
> quite possible that Gather node can occur on inner side of join in
> which case Rescan can be called before even execution starts. This is
> the reason why we have similar check in ExecReScanSeqScan which is
> added during parallel sequential scans (f0661c4e). Does that answer
> your question?

Thanks for the details. got it.

Regards,
Hari Babu
Fujitsu Australia