Function ATPrepDropExpression

src/backend/commands/tablecmds.c:9719–9760

* ALTER TABLE ALTER COLUMN DROP EXPRESSION */

 * ALTER TABLE ALTER COLUMN DROP EXPRESSION
 */
static void
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.
	 */
	if (!recursing)
	{
		HeapTuple	tuple;
		Form_pg_attribute attTup;

		tuple = SearchSysCacheCopyAttName(RelationGetRelid(rel), cmd->name);
		if (!HeapTupleIsValid(tuple))
			ereport(ERROR,
					(errcode(ERRCODE_UNDEFINED_COLUMN),
					 errmsg("column \"%s\" of relation \"%s\" does not exist",
							cmd->name, RelationGetRelationName(rel))));

		attTup = (Form_pg_attribute) GETSTRUCT(tuple);

		if (attTup->attinhcount > 0)
			ereport(ERROR,
					(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
					 errmsg("cannot drop generation expression from inherited column")));
	}
}

/*
 * Return the address of the affected column.