Re: Parallel Seq Scan

On Sat, Oct 17, 2015 at 6:15 AM, Noah Misch <noah(at)leadboat(dot)com> wrote:
>
> On Thu, Oct 15, 2015 at 04:30:01PM +0530, Amit Kapila wrote:
> > On Mon, Oct 12, 2015 at 9:16 PM, Robert Haas <robertmhaas(at)gmail(dot)com>
wrote:
> > > plpgsql_param_fetch() assumes that it can detect whether it's being
> > > called from copyParamList() by checking whether params !=
> > > estate->paramLI. I don't know why this works, but I do know that this
> > > test fails to detect the case where it's being called from
> > > SerializeParamList(), which causes failures in exec_eval_datum() as
> > > predicted. Calls from SerializeParamList() need the same treatment as
> > > calls from copyParamList() because it, too, will try to evaluate every
> > > parameter in the list.
> >
> > From what I understood by looking at code in this area, I think the
check
> > params != estate->paramLI and code under it is required for parameters
> > that are setup by setup_unshared_param_list(). Now unshared params
> > are only created for Cursors and expressions that are passing a R/W
> > object pointer; for cursors we explicitly prohibit the parallel
> > plan generation
> > and I am not sure if it makes sense to generate parallel plans for
> > expressions
> > involving R/W object pointer, if we don't generate parallel plan where
> > expressions involve such parameters, then SerializeParamList() should
not
> > be affected by the check mentioned by you.
>
> The trouble comes from the opposite direction. A
setup_unshared_param_list()
> list is fine under today's code, but a shared param list needs more
help. To
> say it another way, parallel queries that use the shared estate->paramLI
need,
> among other help, the logic now guarded by "params != estate->paramLI".
>

Why would a parallel query need such a logic, that logic is needed mainly
for cursor with params and we don't want a parallelize such cases?

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