Function ATExecDropExpression

src/backend/commands/tablecmds.c:9765–9875

* Return the address of the affected column. */

 * Return the address of the affected column.
 */
static ObjectAddress
ATExecDropExpression(Relation rel, const char *colName, bool missing_ok, LOCKMODE lockmode)
{
	HeapTuple	tuple;
	Form_pg_attribute attTup;
	AttrNumber	attnum;
	Relation	attrelation;
	ObjectAddress address;

	attrelation = table_open(AttributeRelationId, RowExclusiveLock);
	tuple = SearchSysCacheCopyAttName(RelationGetRelid(rel), colName);
	if (!HeapTupleIsValid(tuple))
		ereport(ERROR,
				(errcode(ERRCODE_UNDEFINED_COLUMN),
				 errmsg("column \"%s\" of relation \"%s\" does not exist",
						colName, RelationGetRelationName(rel))));

	attTup = (Form_pg_attribute) GETSTRUCT(tuple);
	attnum = attTup->attnum;

	if (attnum <= 0)
		ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("cannot alter system column \"%s\"",
						colName)));

	if (attTup->attgenerated != ATTRIBUTE_GENERATED_STORED)
	{
		if (!missing_ok)
			ereport(ERROR,
					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
					 errmsg("column \"%s\" of relation \"%s\" is not a stored generated column",
							colName, RelationGetRelationName(rel))));
		else
		{
			ereport(NOTICE,
					(errmsg("column \"%s\" of relation \"%s\" is not a stored generated column, skipping",
							colName, RelationGetRelationName(rel))));
			heap_freetuple(tuple);
			table_close(attrelation, RowExclusiveLock);
			return InvalidObjectAddress;
		}
	}

	attTup->attgenerated = '\0';
	CatalogTupleUpdate(attrelation, &tuple->t_self, tuple);

	InvokeObjectPostAlterHook(RelationRelationId,
							  RelationGetRelid(rel),
							  attTup->attnum);
	ObjectAddressSubSet(address, RelationRelationId,
						RelationGetRelid(rel), attnum);
	heap_freetuple(tuple);

	table_close(attrelation, RowExclusiveLock);

	CommandCounterIncrement();