Hello,
while playing around with the parallel aggregates and seq scan in 9.6beta I noticed that Postgres will stop using parallel plans when cpu_tuple_cost is set to a very small number.
When using the defaults and max_parallel_degree = 4, the following (test) query will be executed with 4 workers
explain (analyze, verbose)
select o.customer_id,
count(*) num_orders,
sum(ol.price) as total_price,
sum(p.purchase_price) as total_purchase_price
from orders o
join order_line ol on o.id = ol.order_id
join product p ON ol.product_id = p.id
group by o.customer_id;
The execution plan is: https://explain.depesz.com/s/C7g
After setting cpu_tuple_cost to something small:
set cpu_tuple_cost = 0.0001;
No parallel wokers are used: https://explain.depesz.com/s/q1zb
I am not sure I understand why this is happening. Why would lowering the CPU cost for a tuple result in not using a parallel plan?
Is this an oversight, a bug or intended?
This is expected. You have modified cost of processing the tuple by CPu but you have not modified the cost of parallel tuple processing (parallel_tuple_cost, if I recall it right). That forces optimizer to evaluate parallel processing of tuples to be more costly, hence chooses a plan without parallel worker.
Perhaps if you reduce the both parameters by same factor/margin, you can see that parallel execution will be preffered when it is of lower cost.
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Best Regards
Sameer Kumar | DB Solution Architect
ASHNIK PTE. LTD.
