Re: Parallel Seq Scan

On Sat, Oct 17, 2015 at 2:15 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> Agreed and on looking at code, I think in below code, if we pass
> parallelOK as true for the cases where Portal is non-null, such a
> problem could happen.
>
> and one such case is
>
> exec_stmt_return_query()
> {
> ..
>
> if (stmt->query != NULL)
>
> {
>
> /* static query */
>
> exec_run_select(estate, stmt->query, 0, &portal, true);
>
> ..
> }
>
>
> In this function we are using controlled fetch mechanism (count as 50) to
> fetch the tuples which we initially thought of not supporting for
> parallelism,
> as such a mechanism is not built for parallel workers and that is the
> reason we want to prohibit cases where ever cursor is getting created.
>
> Do we want to support parallelism for this case on the basis that this API
> will eventually fetch all the tuples by using controlled fetch mechanism?

That was my idea when I made that change, but I think it's not going
to work out well given the way the rest of the code works. Possibly
that should be reverted for now, but maybe only after testing it.

It's worth noting that, as of commit
bfc78d7196eb28cd4e3d6c24f7e607bacecf1129, the consequences of invoking
the executor with a fetch count have been greatly reduced.
Previously, the assumption was that doing that was broken, and if you
did it you got to keep both pieces. But that commit rejiggered things
so that your parallel plan just gets run serially in that case. That
might not be great from a performance perspective, but it beats
undefined behavior by a wide margin. So I suspect that there are some
decisions about where to pass CURSOR_OPT_PARALLEL_OK that need to be
revisited in the light of that change. I haven't had time to do that
yet, but we should do it as soon as we get time.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company