parallel_safe

Hi,

In the comments of add_partial_path, we have:

* We don't generate parameterized partial paths for several reasons. Most
* importantly, they're not safe to execute, because there's nothing to
* make sure that a parallel scan within the parameterized portion of the
* plan is running with the same value in every worker at the same time.

and we are using 'is_parallel_safe(PlannerInfo *root, Node *node)' to see
if it is safe/necessary to generate partial path on a RelOptInfo. In the
code of 'is_parallel_safe':

/*
* We can't pass Params to workers at the moment either, so they are also
* parallel-restricted, unless they are PARAM_EXTERN Params or are
* PARAM_EXEC Params listed in safe_param_ids...
*/
else if (IsA(node, Param))
{
Param *param = (Param *) node;

if (param->paramkind == PARAM_EXTERN)
return false;

if (param->paramkind != PARAM_EXEC ||
!list_member_int(context->safe_param_ids, param->paramid))
{
if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context))
return true;
}
return false; /* nothing to recurse to */
}

Then see the below example:

create table bigt (a int, b int, c int);
insert into bigt select i, i, i from generate_series(1, 1000000)i;
analyze bigt;

select * from bigt o where b = 1;
QUERY PLAN
-----------------------------------
Gather
Workers Planned: 2
-> Parallel Seq Scan on bigt o
Filter: (b = 1)
(4 rows)

select * from bigt o where b = 1 and c = (select sum(c) from bigt i where c = o.c);
QUERY PLAN
-------------------------------------------
Seq Scan on bigt o
Filter: ((b = 1) AND (c = (SubPlan 1)))
SubPlan 1
-> Aggregate
-> Seq Scan on bigt i
Filter: (c = o.c)
(6 rows)

I think the below plan should be correct and more efficiently.

Plan 1:

QUERY PLAN
-------------------------------------------------
Gather
Workers Planned: 2
-> Parallel Seq Scan on bigt o
Filter: ((b = 1) AND (c = (SubPlan 1)))
SubPlan 1
-> Aggregate
-> Seq Scan on bigt
Filter: (c = o.c)
(8 rows)

However the above plan is impossible because:

(1). During the planning of the SubPlan, we use is_parallel_safe() to
set the "bigt i"'s consider_parallel to false because of the above
"PARAM_EXEC" reason.

(2). The parallel_safe of the final SubPlan is set to false due to
rel->consider_parallel.

(3). During the planning of "bigt o", it calls is_parallel_safe and then
it find a subplan->parallel_safe == false, then all the partial path is
impossible.

is_parallel_safe/max_parallel_hazard_walker:

else if (IsA(node, SubPlan))
{
SubPlan *subplan = (SubPlan *) node;
List *save_safe_param_ids;

if (!subplan->parallel_safe &&
max_parallel_hazard_test(PROPARALLEL_RESTRICTED, context))
return true;
...
}

So if we think "plan 1" is valid, then what is wrong?

I think it is better to think about what parallel_safe is designed
for. In Path:

/* OK to use as part of parallel plan? */
bool parallel_safe;

The definition looks to say: the Path/Plan should not be run as a
'parallel_aware' plan, but the code looks to say: The Path/Plan should not be
run in a parallel worker even it is *not* parallel_aware. The reason I
feel the above is because:

* We don't generate parameterized partial paths for several reasons. Most
* importantly, they're not safe to execute, because there's nothing to
* make sure that a parallel scan within the parameterized portion of the
* plan is running with the same value in every worker at the same time.

If a plan which is not parallel-aware, why should we care about the
above stuff?

In the current code, there are some other parallel_safe = false which look
like a *should not be run in a parallel worker rather than parallel
plan*. the cases I know are:
1. path->parallel_safe = false in Gather/GatherMerge.
2. some expressions which is clearly claried as parallel unsafe.

So parallel_safe looks have two different meaning to me. are you feeling
something similar? Do you think treating the different parallel_safe
would make parallel works in some more places? Do you think the benefits
would be beyond the SubPlan one (I can't make a example beside SubPlan
so far).

--
Best Regards
Andy Fan