Re: Fix ALTER TABLE DROP EXPRESSION with inheritance hierarchy

On Wed, Nov 5, 2025 at 2:31 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
>
> If it actually matters for DROP EXPRESSION, then the answer is
> probably "we can't use ATSimpleRecursion for DROP EXPRESSION".
> ATSimpleRecursion is meant for cases where each table can be
> processed independently, regardless of its position in the
> hierarchy.
>

ATPrepAlterColumnType will call ATPrepCmd, which will call again
ATPrepAlterColumnType.
Similarly, we can remove ATSimpleRecursion, and let
ATPrepDropExpression call ATPrepCmd
but that will just be the duplication of ATSimpleRecursion, i think.

/*
* Recurse manually by queueing a new command for each child, if
* necessary. We cannot apply ATSimpleRecursion here because we need to
* remap attribute numbers in the USING expression, if any.
ATPrepAlterColumnType has the above comments.
but here, we don't need to do anything between "rel" and "parent_rel".

ATPrepDropExpression logic is quite simple, it only needs to check the
a. the original source relation is species ONLY or not.
b. the original source relation is inherited column or not.

ALTER TABLE ONLY parent ALTER COLUMN d DROP EXPRESSION;
it will skip ATSimpleRecursion, because first time recurse is
false.(keyword ONLY specified)
the first time enter ATPrepDropExpression, both "recurse" and
"recursing" is false.

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."));

so the above code makes sense to me.
Because we only need to do this check once.

Summary:
1. We need ATSimpleRecursion so that AlteredTableInfo structures for child
relations are populated too, so generated expressions are dropped from all
inherited tables.

2. ATPrepDropExpression only checks the original table specified in the ALTER
TABLE command, and does not apply the check to its child relations.
(for example: ALTER TABLE parent ALTER COLUMN d DROP EXPRESSION;
ATPrepDropExpression check only applies to "parent" not its child relation).