Re: Choosing parallel_degree - Mailing list pgsql-hackers
OK cool, thanks.
Can we remove the minimum size limit when the per table degree setting is applied?
This would help for tables with 2 - 1000 pages combined with a high CPU cost aggregate.
Cheers,
James Sewell,
PostgreSQL Team Lead / Solutions Architect
______________________________________
On Sun, Mar 20, 2016 at 11:23 PM, David Rowley <david.rowley@2ndquadrant.com> wrote:
On 18 March 2016 at 10:13, James Sewell <james.sewell@lisasoft.com> wrote:
> This does bring up an interesting point I don't quite understand though. If I run parallel agg on a table with 4 rows with 2 workers will it run on two workers (2 rows each) or will the first one grab all 4 rows?
It works on a per page basis, workers just each grab the next page to
be scanned from a page counter that sits in shared memory, the worker
just increments the page number, releases the lock on the counter and
scans that page.
See heap_parallelscan_nextpage()
So the answer to your question is probably no. At least not unless the
the page only contained 2 rows.
--
David Rowley http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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