Re: Parallel Seq Scan

On Mon, Oct 12, 2015 at 11:51 AM, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
wrote:
>
> On Mon, Oct 5, 2015 at 11:20 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:
> > For now, I have fixed this by not preserving the startblock incase of
rescan
> > for parallel scan. Note that, I have created a separate patch
> > (parallel_seqscan_heaprescan_v1.patch) for support of rescan (for
parallel
> > scan).
>
> while testing parallel seqscan, My colleague Jing Wang has found a
problem in
> parallel_seqscan_heapscan_v2.patch.
>

Thanks for spotting the issue.

> In function initscan, the allow_sync flag is set to false as the
> number of pages in the
> table are less than NBuffers/4.
>
> if (!RelationUsesLocalBuffers(scan->rs_rd) &&
> scan->rs_nblocks > NBuffers / 4)
>
> As allow_sync flag is false, the function
> heap_parallelscan_initialize_startblock is not
> called in initscan function to initialize the
> parallel_scan->phs_cblock parameter. Because
> of this reason while getting the next page in
> heap_parallelscan_nextpage, it returns
> InvalidBlockNumber, thus it ends the scan without returning the results.
>

Right, it should initialize parallel scan properly even for non-synchronized
scans. Fixed the issue in attached patch. Rebased heap rescan is
attached as well.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com