Install dependencies to prevent dropping partition key columns.

The logic in ATExecDropColumn that rejects dropping partition key
columns is quite an inadequate defense, because it doesn't execute
in cases where a column needs to be dropped due to cascade from
something that only the column, not the whole partitioned table,
depends on.  That leaves us with a badly broken partitioned table;
even an attempt to load its relcache entry will fail.

We really need to have explicit pg_depend entries that show that the
column can't be dropped without dropping the whole table.  Hence,
add those entries.  In v12 and HEAD, bump catversion to ensure that
partitioned tables will have such entries.  We can't do that in
released branches of course, so in v10 and v11 this patch affords
protection only to partitioned tables created after the patch is
installed.  Given the lack of field complaints (this bug was found
by fuzz-testing not by end users), that's probably good enough.

In passing, fix ATExecDropColumn and ATPrepAlterColumnType
messages to be more specific about which partition key column
they're complaining about.

Per report from Manuel Rigger.  Back-patch to v10 where partitioned
tables were added.

Discussion: https://postgr.es/m/CA+u7OA4JKCPFrdrAbOs7XBiCyD61XJxeNav4LefkSmBLQ-Vobg@mail.gmail.com
Discussion: https://postgr.es/m/31920.1562526703@sss.pgh.pa.us

src/test/regress/expected/alter_table.out

@@ -3446,13 +3446,13 @@ LINE 1: ALTER TABLE partitioned ADD EXCLUDE USING gist (a WITH &&);
                                     ^
 -- cannot drop column that is part of the partition key
 ALTER TABLE partitioned DROP COLUMN a;
-ERROR:  cannot drop column named in partition key
+ERROR:  cannot drop column "a" because it is part of the partition key of relation "partitioned"
 ALTER TABLE partitioned ALTER COLUMN a TYPE char(5);
-ERROR:  cannot alter type of column named in partition key
+ERROR:  cannot alter column "a" because it is part of the partition key of relation "partitioned"
 ALTER TABLE partitioned DROP COLUMN b;
-ERROR:  cannot drop column referenced in partition key expression
+ERROR:  cannot drop column "b" because it is part of the partition key of relation "partitioned"
 ALTER TABLE partitioned ALTER COLUMN b TYPE char(5);
-ERROR:  cannot alter type of column referenced in partition key expression
+ERROR:  cannot alter column "b" because it is part of the partition key of relation "partitioned"
 -- partitioned table cannot participate in regular inheritance
 CREATE TABLE nonpartitioned (
 	a int,
@@ -3945,9 +3945,9 @@ ERROR:  cannot change inheritance of a partition
 -- partitioned tables; for example, part_5, which is list_parted2's
 -- partition, is partitioned on b;
 ALTER TABLE list_parted2 DROP COLUMN b;
-ERROR:  cannot drop column named in partition key
+ERROR:  cannot drop column "b" because it is part of the partition key of relation "part_5"
 ALTER TABLE list_parted2 ALTER COLUMN b TYPE text;
-ERROR:  cannot alter type of column named in partition key
+ERROR:  cannot alter column "b" because it is part of the partition key of relation "part_5"
 -- dropping non-partition key columns should be allowed on the parent table.
 ALTER TABLE list_parted DROP COLUMN b;
 SELECT * FROM list_parted;

src/test/regress/expected/create_table.out

@@ -501,6 +501,42 @@ Partition of: partitioned2 FOR VALUES FROM ('-1', 'aaaaa') TO (100, 'ccccc')
 Partition constraint: (((a + 1) IS NOT NULL) AND (substr(b, 1, 5) IS NOT NULL) AND (((a + 1) > '-1'::integer) OR (((a + 1) = '-1'::integer) AND (substr(b, 1, 5) >= 'aaaaa'::text))) AND (((a + 1) < 100) OR (((a + 1) = 100) AND (substr(b, 1, 5) < 'ccccc'::text))))
 
 DROP TABLE partitioned, partitioned2;
+-- check that dependencies of partition columns are handled correctly
+create domain intdom1 as int;
+create table partitioned (
+	a intdom1,
+	b text
+) partition by range (a);
+alter table partitioned drop column a;  -- fail
+ERROR:  cannot drop column "a" because it is part of the partition key of relation "partitioned"
+drop domain intdom1;  -- fail, requires cascade
+ERROR:  cannot drop type intdom1 because other objects depend on it
+DETAIL:  table partitioned depends on type intdom1
+HINT:  Use DROP ... CASCADE to drop the dependent objects too.
+drop domain intdom1 cascade;
+NOTICE:  drop cascades to table partitioned
+table partitioned;  -- gone
+ERROR:  relation "partitioned" does not exist
+LINE 1: table partitioned;
+              ^
+-- likewise for columns used in partition expressions
+create domain intdom1 as int;
+create table partitioned (
+	a intdom1,
+	b text
+) partition by range (plusone(a));
+alter table partitioned drop column a;  -- fail
+ERROR:  cannot drop column "a" because it is part of the partition key of relation "partitioned"
+drop domain intdom1;  -- fail, requires cascade
+ERROR:  cannot drop type intdom1 because other objects depend on it
+DETAIL:  table partitioned depends on type intdom1
+HINT:  Use DROP ... CASCADE to drop the dependent objects too.
+drop domain intdom1 cascade;
+NOTICE:  drop cascades to table partitioned
+table partitioned;  -- gone
+ERROR:  relation "partitioned" does not exist
+LINE 1: table partitioned;
+              ^
 --
 -- Partitions
 --

src/test/regress/sql/create_table.sql

@@ -449,6 +449,39 @@ CREATE TABLE part2_1 PARTITION OF partitioned2 FOR VALUES FROM (-1, 'aaaaa') TO
 
 DROP TABLE partitioned, partitioned2;
 
+-- check that dependencies of partition columns are handled correctly
+create domain intdom1 as int;
+
+create table partitioned (
+	a intdom1,
+	b text
+) partition by range (a);
+
+alter table partitioned drop column a;  -- fail
+
+drop domain intdom1;  -- fail, requires cascade
+
+drop domain intdom1 cascade;
+
+table partitioned;  -- gone
+
+-- likewise for columns used in partition expressions
+create domain intdom1 as int;
+
+create table partitioned (
+	a intdom1,
+	b text
+) partition by range (plusone(a));
+
+alter table partitioned drop column a;  -- fail
+
+drop domain intdom1;  -- fail, requires cascade
+
+drop domain intdom1 cascade;
+
+table partitioned;  -- gone
+
+
 --
 -- Partitions
 --