Tom Lane schrieb am 20.06.2018 um 16:03:
>> Consider the following execution plan:
>> ...
>> -> Aggregate (cost=26.87..26.87 rows=1 width=32) (actual time=0.012..0.012 rows=1 loops=700000)
>> -> Bitmap Heap Scan on orders o2 (cost=3.45..26.85 rows=8 width=8) (actual time=0.004..0.008 rows=8 loops=700000)
>> -> Bitmap Index Scan on orders_customer_id_order_date_idx (cost=0.00..3.45 rows=8 width=0) (actual time=0.003..0.003 rows=8 loops=700000)
>
>> My expectation would have been that the "Aggregate" step shows the actual time as a product of the number of loops.
>
> No, that looks fine to me. The rule of thumb for reading this is total
> time spent in/below this node is "actual time" times "number of loops".
OK, if that is the rule I can live with that ;)
> It seems a bit odd that the Agg node would account for a third of the
> total execution time when it's only processing 8 rows on average ...
> but maybe it's a really expensive aggregate.
But it's processing those 8 rows 700.000 times - so the total time seems correct.
FWIW, the query looks like this:
select customer_id,
amount,
sales_person_id
from orders o1
where amount = (select max(o2.amount)
from orders o2
where o2.customer_id = o1.customer_id);
It's not a real world query - it's just there to illustrate the drawbacks of co-related sub-queries.
> Another thought is that the EXPLAIN ANALYZE instrumentation itself
> can account for significant per-node-invocation overhead. If the
> total execution time drops significantly when you add "timing off"
> to the EXPLAIN options, then that's probably a factor in making
> the Agg node look relatively expensive.
"timing off" doesn't really change the execution time (it's about 60ms faster without)
Thanks for the answer, I am not really concerned about the query performance itself, just about the plan ;)
Thomas
