Turning off enable_partition_pruning doesn't

I'd have thought that disabling enable_partition_pruning would,
um, disable partition pruning. It does not:

regression=# create table p (a int) partition by list (a);
CREATE TABLE
regression=# create table p1 partition of p for values in (1);
CREATE TABLE
regression=# create table p2 partition of p for values in (2);
CREATE TABLE
regression=# explain select * from p1 where a = 3;
QUERY PLAN
----------------------------------------------------
Seq Scan on p1 (cost=0.00..41.88 rows=13 width=4)
Filter: (a = 3)
(2 rows)

regression=# set enable_partition_pruning TO off;
SET
regression=# explain select * from p1 where a = 3;
QUERY PLAN
----------------------------------------------------
Seq Scan on p1 (cost=0.00..41.88 rows=13 width=4)
Filter: (a = 3)
(2 rows)

The fact that we fail to prune the first child is a separate issue
driven by some ruleutils.c limitations, cf
https://www.postgresql.org/message-id/flat/001001d4f44b$2a2cca50$7e865ef0$(at)lab(dot)ntt(dot)co(dot)jp

My point here is that the second EXPLAIN should have shown scanning
both partitions, shouldn't it?

(v11 behaves the same as HEAD here; didn't try v10.)

regards, tom lane