parallelize queries containing initplans

From: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
To: pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: parallelize queries containing initplans
Date: 2016-12-28 11:50:50
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By seeing the subject line, one might wonder why we need to consider
parallelizing the queries containing initplans differently from
queries containing subplans considering that I have posted a mail to
achieve later a few hours back. The reason is that both are treated
differently with respect to parallelism and otherwise as well and both
can be parallelized in a different way depending on the design we
choose. InitPlans can be used in three forms (a) a Param node
representing a single scalar result (b) a row comparison tree
containing multiple Param nodes (c) NULL constant for MULTIEXPR
subquery whereas SubPlans are used as SubPlan nodes. Here, I am
primarily interested in parallelizing queries that contain InitPlans
of the form (a) and the reason is that I have seen that form used more
as compared to other forms (primarily based on a study of TPC-H and
TPC-DS workloads). However, if we find that parallelizing other forms
can be done along with it easily, then that is excellent. To start
with let us see the plan of TPC-H query (Q-22) and understand how it
can be improved.

InitPlan 1 (returns $0)
-> Finalize Aggregate
-> Gather
Workers Planned: 2
-> Partial Aggregate
-> Parallel Seq Scan on customer customer_1
Filter: (...)
-> GroupAggregate
Group Key: ("substring"((customer.c_phone)::text, 1, 2))
-> Sort
Sort Key: ("substring"((customer.c_phone)::text, 1, 2))
-> Nested Loop Anti Join
-> Seq Scan on customer
Filter: ((c_acctbal > $0) AND (...)))
-> Index Only Scan using idx_orders_custkey on orders
Index Cond: (o_custkey = customer.c_custkey)

In the above plan, we can see that the join on customer and orders
table (Nested Loop Anti Join) is not parallelised even though we have
the capability to parallelize Nested Loop Joins. The reason for not
choosing the parallel plan is that one of the nodes (Seq Scan on
customer) is referring to initplan and we consider such nodes as
parallel-restricted which means they can't be parallelised. Now, I
could see three ways of parallelizing such a query. The first way is
that we just push parallel-safe initplans to workers and allow them to
execute it, the drawback of this approach is that it won't be able to
push initplans in cases as shown above where initplan is
parallel-unsafe (contains Gather node) and second is we will lose the
expectation of single evaluation. The second way is that we always
execute the initplan in the master backend and pass the resultant
value to the worker, this will allow above form of plans to push
initplans to workers and hence can help in enabling parallelism for
other nodes in plan tree. The drawback of the second approach is
that we need to evaluate the initplan before it is actually required
which means that we might evaluate it even when it is not required. I
am not sure if it is always safe to assume that we can evaluate the
initplan before pushing it to workers especially for the cases when it
is far enough down in the plan tree which we are parallelizing,
however, I think we can assume it when the iniplan is above the plan
tree where it is used (like in the above case). The third way is that
we allow Gather node to be executed below another Gather node, but I
think that will be bad especially for the plans like above because
each worker needs to further spawn another set of workers to evaluate
the iniplan which could be done once. Now we can build some way such
that only one of the workers executes such an initplan and share the
values with other workers, but I think overall this requires much more
effort than first or second approach.

Among all the three approaches, first seems to be simpler than the
other two, but I feel if we just do that then we leave a lot on the
table. Another way to accomplish this project could be that we do a
mix of first and second such that when the initplan is above the plan
tree to be parallelized, then use the second approach (one-time
evaluation by master backend and share the result with workers),
otherwise use the first approach of pushing down the initplan to


With Regards,
Amit Kapila.


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