Function ATExecDropNotNull

src/backend/commands/tablecmds.c:8997–9138

* Return the address of the modified column. If the column was already * nullable, InvalidObjectAddress is returned. */

 * nullable, InvalidObjectAddress is returned.
 */
static ObjectAddress
ATExecDropNotNull(Relation rel, const char *colName, LOCKMODE lockmode)
{
	HeapTuple	tuple;
	Form_pg_attribute attTup;
	AttrNumber	attnum;
	Relation	attr_rel;
	List	   *indexoidlist;
	ListCell   *indexoidscan;
	ObjectAddress address;

	/*
	 * lookup the attribute
	 */
	attr_rel = 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;

	/* Prevent them from altering a system attribute */
	if (attnum <= 0)
		ereport(ERROR,
				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
				 errmsg("cannot alter system column \"%s\"",
						colName)));

	if (attTup->attidentity)
		ereport(ERROR,
				(errcode(ERRCODE_SYNTAX_ERROR),
				 errmsg("column \"%s\" of relation \"%s\" is an identity column",
						colName, RelationGetRelationName(rel))));

	/*
	 * Check that the attribute is not in a primary key or in an index used as
	 * a replica identity.
	 *
	 * Note: we'll throw error even if the pkey index is not valid.
	 */

	/* Loop over all indexes on the relation */
	indexoidlist = RelationGetIndexList(rel);

	foreach(indexoidscan, indexoidlist)
	{
		Oid			indexoid = lfirst_oid(indexoidscan);
		HeapTuple	indexTuple;
		Form_pg_index indexStruct;
		int			i;

		indexTuple = SearchSysCache1(INDEXRELID, ObjectIdGetDatum(indexoid));
		if (!HeapTupleIsValid(indexTuple))
			elog(ERROR, "cache lookup failed for index %u", indexoid);