Re: Enabling parallelism for queries coming from SQL or other PL functions

On Thu, Mar 23, 2017 at 12:41 AM, Rafia Sabih
<rafia(dot)sabih(at)enterprisedb(dot)com> wrote:
> Agree, done.

OK, committed execute-once-v3.patch after some further study. See
also https://www.postgresql.org/message-id/CA%2BTgmoZ_ZuH%2BauEeeWnmtorPsgc_SmP%2BXWbDsJ%2BcWvWBSjNwDQ%40mail.gmail.com
which resulted from that study.

Regarding pl_parallel_opt_support_v3.patch, the change to
init_execution_state looks good. Whether or not it's safe to use
parallel query doesn't depend on whether the function is marked
volatile, so the current code is just silly (and, arguably,
readonly_func is misnamed). The changes to spi.c also seem fine; both
of those functions execute the plan to completion and don't allow
cursor operations, so we're good.

The changes to the plpgsql code don't look so good to me. The change
to exec_stmt_return_query fixes the same bug that I mentioned in the
email linked above, but only half of it -- it corrects the RETURN
QUERY EXECUTE case but not the RETURN QUERY case. And it's clearly a
separate change; that part is a bug fix, not an enhancement. Some of
the other changes depend on whether we're in a trigger, which seems
irrelevant to whether we can use parallelism. Even if the outer query
is doing writes, we can still use parallelism for queries inside the
trigger function if warranted. It's probably a rare case to have
queries inside a trigger that are expensive enough to justify such
handling but I don't see the value of putting in special-case logic to
prevent it.

I suspect that code fails to achieve its goals anyway. At the top of
exec_eval_expr(), you call exec_prepare_plan() and unconditionally
pass CURSOR_OPT_PARALLEL_OK, so when that function returns, expr->plan
might now be a parallel plan. If we reach the call to
exec_run_select() further down in that function, and if we happen to
pass false, it's not going to matter, because exec_run_select() is
going to find the plan already initialized.

--
Robert Haas
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