Re: Fix ALTER TABLE DROP EXPRESSION with inheritance hierarchy

Hi all,

I tried to fix a bug in PostgreSQL where ALTER TABLE ... DROP EXPRESSION
fails on multi-level inheritance hierarchies.

Bug:
When a parent table has a generated column and child/grandchild tables
inherit from it, executing:

ALTER TABLE parent ALTER COLUMN d DROP EXPRESSION;

ERROR: ALTER TABLE / DROP EXPRESSION must be applied to child tables too

Fix Details:

-

Updated file: src/backend/commands/tablecmds.c

-

Function modified: ATPrepDropExpression()

-

Change: Added !recursing check to ensure proper recursion across
inheritance.

if (!recurse && !recursing &&

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"),

errhint("Do not specify the ONLY keyword."));

-

Test Query DROP TABLE IF EXISTS parent CASCADE;
CREATE TABLE parent (a int, d int GENERATED ALWAYS AS (11) STORED);
CREATE TABLE child () INHERITS (parent);
CREATE TABLE grandchild () INHERITS (child);
ALTER TABLE parent ALTER COLUMN d DROP EXPRESSION;
-

Output ALTER TABLE

On Thu, Aug 28, 2025 at 10:49 AM Kirill Reshke <reshkekirill(at)gmail(dot)com>
wrote:

> On Thu, 28 Aug 2025 at 08:35, jian he <jian(dot)universality(at)gmail(dot)com> wrote:
> >
> > That means, we don't need to change the ATPrepDropExpression function
> > argument for now?
>
> Sure. V3 with this attached, and I think we can move cf entry to RFC
>
> --
> Best regards,
> Kirill Reshke
>