Function CheckTableNotInUse

src/backend/commands/tablecmds.c:4707–4739

* Disallow ALTER TABLE (and similar commands) when the current backend has * any open reference to the target table besides the one just acquired by * the calling command; this implies there's an open cursor or active plan. * We need this check because our lock doesn't protect us against stomping * on our own foot, only other people's feet! * * For ALTER TABLE, the only case known to cause s

 * The statement name (eg, "ALTER TABLE") is passed for use in error messages.
 */
void
CheckTableNotInUse(Relation rel, const char *stmt)
{
	int			expected_refcnt;

	expected_refcnt = rel->rd_isnailed ? 2 : 1;

	/*
	 * XXX For a bitmap index, since vacuum (or vacuum full) is currently done through
	 * reindex_index, the reference count could be 2 (or 3). We set it
	 * here until vacuum is done properly.
	 */
	if (expected_refcnt == 1 &&
		RelationIsBitmapIndex(rel) &&
		(rel->rd_refcnt == 2 || rel->rd_refcnt == 3))
		expected_refcnt = rel->rd_refcnt;

	if (rel->rd_refcnt != expected_refcnt)
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_IN_USE),
		/* translator: first %s is a SQL command, eg ALTER TABLE */
				 errmsg("cannot %s \"%s\" because it is being used by active queries in this session",
						stmt, RelationGetRelationName(rel))));

	if (rel->rd_rel->relkind != RELKIND_INDEX &&
		rel->rd_rel->relkind != RELKIND_PARTITIONED_INDEX &&
		AfterTriggerPendingOnRel(RelationGetRelid(rel)))
		ereport(ERROR,
				(errcode(ERRCODE_OBJECT_IN_USE),
		/* translator: first %s is a SQL command, eg ALTER TABLE */
				 errmsg("cannot %s \"%s\" because it has pending trigger events",
						stmt, RelationGetRelationName(rel))));
}

/*
 * AlterTableLookupRelation