Fix dependency handling of column drop with partitioned tables

When dropping a column on a partitioned table which has one or more
partitioned indexes, the operation was failing as dependencies with
partitioned indexes using the column dropped were not getting removed in
a way consistent with the columns involved across all the relations part
of an inheritance tree.

This commit refactors the code executing column drop so as all the
columns from an inheritance tree to remove are gathered first, and
dropped all at the end.  This way, we let the dependency machinery sort
out by itself the deletion of all the columns with the partitioned
indexes across a partition tree.

This issue has been introduced by 1d92a0c, so backpatch down to
REL_12_STABLE.

Author: Amit Langote, Michael Paquier
Reviewed-by: Álvaro Herrera, Ashutosh Sharma
Discussion: https://postgr.es/m/CA+HiwqE9kuBsZ3b5pob2-cvE8ofzPWs-og+g8bKKGnu6b4-yTQ@mail.gmail.com
Backpatch-through: 12

src/test/regress/expected/indexing.out

@@ -1258,3 +1258,64 @@ ERROR:  cannot drop inherited constraint "parted_uniq_detach_test1_a_key" of rel
 alter table parted_uniq_detach_test detach partition parted_uniq_detach_test1;
 alter table parted_uniq_detach_test1 drop constraint parted_uniq_detach_test1_a_key;
 drop table parted_uniq_detach_test, parted_uniq_detach_test1;
+-- check that dropping a column takes with it any partitioned indexes
+-- depending on it.
+create table parted_index_col_drop(a int, b int, c int)
+  partition by list (a);
+create table parted_index_col_drop1 partition of parted_index_col_drop
+  for values in (1) partition by list (a);
+-- leave this partition without children.
+create table parted_index_col_drop2 partition of parted_index_col_drop
+  for values in (2) partition by list (a);
+create table parted_index_col_drop11 partition of parted_index_col_drop1
+  for values in (1);
+create index on parted_index_col_drop (b);
+create index on parted_index_col_drop (c);
+create index on parted_index_col_drop (b, c);
+alter table parted_index_col_drop drop column c;
+\d parted_index_col_drop
+ Partitioned table "public.parted_index_col_drop"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Partition key: LIST (a)
+Indexes:
+    "parted_index_col_drop_b_idx" btree (b)
+Number of partitions: 2 (Use \d+ to list them.)
+
+\d parted_index_col_drop1
+ Partitioned table "public.parted_index_col_drop1"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Partition of: parted_index_col_drop FOR VALUES IN (1)
+Partition key: LIST (a)
+Indexes:
+    "parted_index_col_drop1_b_idx" btree (b)
+Number of partitions: 1 (Use \d+ to list them.)
+
+\d parted_index_col_drop2
+ Partitioned table "public.parted_index_col_drop2"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Partition of: parted_index_col_drop FOR VALUES IN (2)
+Partition key: LIST (a)
+Indexes:
+    "parted_index_col_drop2_b_idx" btree (b)
+Number of partitions: 0
+
+\d parted_index_col_drop11
+      Table "public.parted_index_col_drop11"
+ Column |  Type   | Collation | Nullable | Default 
+--------+---------+-----------+----------+---------
+ a      | integer |           |          | 
+ b      | integer |           |          | 
+Partition of: parted_index_col_drop1 FOR VALUES IN (1)
+Indexes:
+    "parted_index_col_drop11_b_idx" btree (b)
+
+drop table parted_index_col_drop;

src/test/regress/sql/indexing.sql

@@ -703,3 +703,24 @@ alter table parted_uniq_detach_test1 drop constraint parted_uniq_detach_test1_a_
 alter table parted_uniq_detach_test detach partition parted_uniq_detach_test1;
 alter table parted_uniq_detach_test1 drop constraint parted_uniq_detach_test1_a_key;
 drop table parted_uniq_detach_test, parted_uniq_detach_test1;
+
+-- check that dropping a column takes with it any partitioned indexes
+-- depending on it.
+create table parted_index_col_drop(a int, b int, c int)
+  partition by list (a);
+create table parted_index_col_drop1 partition of parted_index_col_drop
+  for values in (1) partition by list (a);
+-- leave this partition without children.
+create table parted_index_col_drop2 partition of parted_index_col_drop
+  for values in (2) partition by list (a);
+create table parted_index_col_drop11 partition of parted_index_col_drop1
+  for values in (1);
+create index on parted_index_col_drop (b);
+create index on parted_index_col_drop (c);
+create index on parted_index_col_drop (b, c);
+alter table parted_index_col_drop drop column c;
+\d parted_index_col_drop
+\d parted_index_col_drop1
+\d parted_index_col_drop2
+\d parted_index_col_drop11
+drop table parted_index_col_drop;