On Wed, Oct 09, 2019 at 06:36:35AM -0300, Alvaro Herrera wrote:
> Right, something like that.  Needs a comment to explain what we do and
> how recursing=true correlates with addrs=NULL, I think.  Maybe add an
> assert.

Yes, that would be a thing to do.  So I have added more comments
regarding that aspect, an assertion, and more tests with a partitioned
table without any children, and an actual check that all columns have
been dropped in the leaves of the partition tree.  How does that look?
--
Michael
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 05593f3316..dd5f9cad68 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -401,7 +401,8 @@ static void ATPrepDropColumn(List **wqueue, Relation rel, bool recurse, bool rec
 static ObjectAddress ATExecDropColumn(List **wqueue, Relation rel, const char *colName,
 									  DropBehavior behavior,
 									  bool recurse, bool recursing,
-									  bool missing_ok, LOCKMODE lockmode);
+									  bool missing_ok, LOCKMODE lockmode,
+									  ObjectAddresses *addrs);
 static ObjectAddress ATExecAddIndex(AlteredTableInfo *tab, Relation rel,
 									IndexStmt *stmt, bool is_rebuild, LOCKMODE lockmode);
 static ObjectAddress ATExecAddConstraint(List **wqueue,
@@ -4273,12 +4274,14 @@ ATExecCmd(List **wqueue, AlteredTableInfo *tab, Relation rel,
 		case AT_DropColumn:		/* DROP COLUMN */
 			address = ATExecDropColumn(wqueue, rel, cmd->name,
 									   cmd->behavior, false, false,
-									   cmd->missing_ok, lockmode);
+									   cmd->missing_ok, lockmode,
+									   NULL);
 			break;
 		case AT_DropColumnRecurse:	/* DROP COLUMN with recursion */
 			address = ATExecDropColumn(wqueue, rel, cmd->name,
 									   cmd->behavior, true, false,
-									   cmd->missing_ok, lockmode);
+									   cmd->missing_ok, lockmode,
+									   NULL);
 			break;
 		case AT_AddIndex:		/* ADD INDEX */
 			address = ATExecAddIndex(tab, rel, (IndexStmt *) cmd->def, false,
@@ -7004,12 +7007,16 @@ ATPrepDropColumn(List **wqueue, Relation rel, bool recurse, bool recursing,
 
 /*
  * Return value is the address of the dropped column.
+ *
+ * *addrs stores the object addresses of all the columns to delete in
+ * case of a recursive lookup on children relations.
  */
 static ObjectAddress
 ATExecDropColumn(List **wqueue, Relation rel, const char *colName,
 				 DropBehavior behavior,
 				 bool recurse, bool recursing,
-				 bool missing_ok, LOCKMODE lockmode)
+				 bool missing_ok, LOCKMODE lockmode,
+				 ObjectAddresses *addrs)
 {
 	HeapTuple	tuple;
 	Form_pg_attribute targetatt;
@@ -7022,6 +7029,16 @@ ATExecDropColumn(List **wqueue, Relation rel, const char *colName,
 	if (recursing)
 		ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);
 
+	/*
+	 * Initialize the location of addresses which will be deleted on a
+	 * recursive lookup on children relations.  The structure to store all the
+	 * object addresses to delete is initialized once when the recursive
+	 * lookup begins.
+	 */
+	Assert(!recursing || addrs != NULL);
+	if (!recursing)
+		addrs = new_object_addresses();
+
 	/*
 	 * get the number of the attribute
 	 */
@@ -7134,7 +7151,7 @@ ATExecDropColumn(List **wqueue, Relation rel, const char *colName,
 					/* Time to delete this child column, too */
 					ATExecDropColumn(wqueue, childrel, colName,
 									 behavior, true, true,
-									 false, lockmode);
+									 false, lockmode, addrs);
 				}
 				else
 				{
@@ -7170,14 +7187,20 @@ ATExecDropColumn(List **wqueue, Relation rel, const char *colName,
 		table_close(attr_rel, RowExclusiveLock);
 	}
 
-	/*
-	 * Perform the actual column deletion
-	 */
+	/* Add object to delete */
 	object.classId = RelationRelationId;
 	object.objectId = RelationGetRelid(rel);
 	object.objectSubId = attnum;
+	add_exact_object_address(&object, addrs);
 
-	performDeletion(&object, behavior, 0);
+	/*
+	 * The recursion is ending, hence perform the actual column deletions.
+	 */
+	if (!recursing)
+	{
+		performMultipleDeletions(addrs, behavior, 0);
+		free_object_addresses(addrs);
+	}
 
 	return object;
 }
diff --git a/src/test/regress/expected/indexing.out b/src/test/regress/expected/indexing.out
index c143df5114..73988c881d 100644
--- a/src/test/regress/expected/indexing.out
+++ b/src/test/regress/expected/indexing.out
@@ -1258,3 +1258,54 @@ 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, 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)
+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)
+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)
+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)
+
+drop table parted_index_col_drop;
diff --git a/src/test/regress/sql/indexing.sql b/src/test/regress/sql/indexing.sql
index cc3d0abfb7..45eeac3dd6 100644
--- a/src/test/regress/sql/indexing.sql
+++ b/src/test/regress/sql/indexing.sql
@@ -703,3 +703,22 @@ 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, 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;

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