Re: Parallel Seq Scan

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".