How to explain a query in PostgreSQL

If you are using partitioned tables, make sure you have enabled partition pruning:

SET enable_partition_pruning to on;

The explain statement is simple enough:

explain select * from documents where dokumenttype='SUBPOENA';

                            QUERY PLAN
---------------------------------------------------------------------
 Append  (cost=0.00..1.02 rows=1 width=774)
   ->  Seq Scan on P_SUBPOENA  (cost=0.00..1.01 rows=1 width=774)
         Filter: ((documenttype)::text = 'SUBPOENA'::text)
(3 rows)

Since this is the LIST-partitioned table outlined in this post, I know the optimizer picked the correct partition for my predicate.

For the LIST-range subpartitioned table outlined in this post, I get the following query plan:

explain select * from orders where country_code='se' and order_total between 4000 and 4999;
                                                           QUERY PLAN
---------------------------------------------------------------------------------------------------------------------------------
 Append  (cost=0.00..28.21 rows=1 width=50)
   ->  Seq Scan on large_orders_se  (cost=0.00..28.20 rows=1 width=50)
         Filter: ((order_total >= '4000'::numeric) AND (order_total <= '4999'::numeric) AND ((country_code)::text = 'se'::text))
(3 rows)

Regarding parallelism, the documentation tells you to watch for the terms Gather or Gather Merge Node in the query plan:
An example:

"Subquery Scan on andre  (cost=1000.00..10024950.78 rows=294885 width=84)"
"  ->  Gather  (cost=1000.00..10013892.60 rows=294885 width=64)"
"        Workers Planned: 2"
"        ->  ProjectSet  (cost=0.00..9983404.10 rows=29488500 width=64)"
"              ->  ProjectSet  (cost=0.00..9830063.90 rows=12286900 width=128)"