Disallow ALTER TABLE ONLY / DROP EXPRESSION

The current implementation cannot handle this correctly, so just
forbid it for now.

GENERATED clauses must be attached to the column definition and cannot
be added later like DEFAULT, so if a child table has a generation
expression that the parent does not have, the child column will
necessarily be an attlocal column.  So to implement ALTER TABLE ONLY /
DROP EXPRESSION, we'd need extra code to update attislocal of the
direct child tables, somewhat similar to how DROP COLUMN does it, so
that the resulting state can be properly dumped and restored.

Discussion: https://www.postgresql.org/message-id/flat/15830.1575468847%40sss.pgh.pa.us

src/backend/commands/tablecmds.c

@@ -413,7 +413,7 @@ static ObjectAddress ATExecAddIdentity(Relation rel, const char *colName,
 static ObjectAddress ATExecSetIdentity(Relation rel, const char *colName,
 									   Node *def, LOCKMODE lockmode);
 static ObjectAddress ATExecDropIdentity(Relation rel, const char *colName, bool missing_ok, LOCKMODE lockmode);
-static void ATPrepDropExpression(Relation rel, AlterTableCmd *cmd, bool recursing);
+static void ATPrepDropExpression(Relation rel, AlterTableCmd *cmd, bool recurse, bool recursing, LOCKMODE lockmode);
 static ObjectAddress ATExecDropExpression(Relation rel, const char *colName, bool missing_ok, LOCKMODE lockmode);
 static ObjectAddress ATExecSetStatistics(Relation rel, const char *colName, int16 colNum,
 										 Node *newValue, LOCKMODE lockmode);
@@ -4151,7 +4151,7 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 		case AT_DropExpression: /* ALTER COLUMN DROP EXPRESSION */
 			ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);
 			ATSimpleRecursion(wqueue, rel, cmd, recurse, lockmode, context);
-			ATPrepDropExpression(rel, cmd, recursing);
+			ATPrepDropExpression(rel, cmd, recurse, recursing, lockmode);
 			pass = AT_PASS_DROP;
 			break;
 		case AT_SetStatistics:	/* ALTER COLUMN SET STATISTICS */
@@ -7262,8 +7262,24 @@ ATExecDropIdentity(Relation rel, const char *colName, bool missing_ok, LOCKMODE
  * ALTER TABLE ALTER COLUMN DROP EXPRESSION
  */
 static void
-ATPrepDropExpression(Relation rel, AlterTableCmd *cmd, bool recursing)
+ATPrepDropExpression(Relation rel, AlterTableCmd *cmd, bool recurse, bool recursing, LOCKMODE lockmode)
 {
+	/*
+	 * Reject ONLY if there are child tables.  We could implement this, but it
+	 * is a bit complicated.  GENERATED clauses must be attached to the column
+	 * definition and cannot be added later like DEFAULT, so if a child table
+	 * has a generation expression that the parent does not have, the child
+	 * column will necessarily be an attlocal column.  So to implement ONLY
+	 * here, we'd need extra code to update attislocal of the direct child
+	 * tables, somewhat similar to how DROP COLUMN does it, so that the
+	 * resulting state can be properly dumped and restored.
+	 */
+	if (!recurse &&
+		find_inheritance_children(RelationGetRelid(rel), lockmode))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("ALTER TABLE / DROP EXPRESSION must be applied to child tables too")));
+
 	/*
 	 * Cannot drop generation expression from inherited columns.
 	 */

src/test/regress/expected/generated.out

@@ -805,13 +805,14 @@ CREATE TABLE gtest30 (
     b int GENERATED ALWAYS AS (a * 2) STORED
 );
 CREATE TABLE gtest30_1 () INHERITS (gtest30);
-ALTER TABLE ONLY gtest30 ALTER COLUMN b DROP EXPRESSION;
+ALTER TABLE ONLY gtest30 ALTER COLUMN b DROP EXPRESSION;  -- error
+ERROR:  ALTER TABLE / DROP EXPRESSION must be applied to child tables too
 \d gtest30
                             Table "public.gtest30"
  Column |  Type   | Collation | Nullable |              Default               
---------+---------+-----------+----------+---------
+--------+---------+-----------+----------+------------------------------------
  a      | integer |           |          | 
- b      | integer |           |          | 
+ b      | integer |           |          | generated always as (a * 2) stored
 Number of child tables: 1 (Use \d+ to list them.)
 
 \d gtest30_1

src/test/regress/sql/generated.sql

@@ -411,7 +411,7 @@ CREATE TABLE gtest30 (
     b int GENERATED ALWAYS AS (a * 2) STORED
 );
 CREATE TABLE gtest30_1 () INHERITS (gtest30);
-ALTER TABLE ONLY gtest30 ALTER COLUMN b DROP EXPRESSION;
+ALTER TABLE ONLY gtest30 ALTER COLUMN b DROP EXPRESSION;  -- error
 \d gtest30
 \d gtest30_1
 ALTER TABLE gtest30_1 ALTER COLUMN b DROP EXPRESSION;  -- error