On Tue, May 31, 2016 at 2:22 PM, Josh berkus <josh@agliodbs.com> wrote:
On 05/31/2016 11:17 AM, Peter Eisentraut wrote: > On 5/31/16 2:02 PM, Josh berkus wrote: >> I get where you're coming from, but I think Haas's query plan output is >> going to show us the confusion we're going to get. So we need to either >> change the parameter, the explain output, or brace ourselves for endless >> repeated questions. > > Changing the explain output doesn't sound so bad to me. > > The users' problem is that the parameter setting ought to match the > EXPLAIN output. > > The developers' problem is that the EXPLAIN output actually corresponds > to leader + (N-1) workers internally. > > I think we can hope that developers are going to be less confused about > that than users.
Makes sense.
One more consistency question: what's the effect of running out of max_parallel_workers?
That is, say max_parallel_workers is set to 10, and 8 are already allocated. If I ask for max_parallel_X = 4, how many cores to I use?
Presumably the leader isn't counted towards max_parallel_workers?
You'd have three O/S processes - the one dedicated to your session and you'd pick up two additional processes from the worker pool to assist.
How the O/S assigns those to cores is outside PostgreSQL's jurisdiction.