多列上的DISTINCT查询无效 - Google App引擎数据存储区

I want to fetch distinct multiple columns from Google app engine Datastore in endpoint class. For that i am using below code but problem is that if i try to fetch only single column then only it works properly and returns DISTINCT data. If i try to fetch multiple columns then DISTINCT doesn't work and it returns all records without any effect of Distinct query. I don't know what is wrong with my code. Please suggest any solution and guide me where i am going wrong. Thank you.

我想从端点类中的Google应用引擎数据存储区中获取不同的多个列。为此，我使用下面的代码，但问题是，如果我尝试只获取单列，那么只有它正常工作并返回DISTINCT数据。如果我尝试获取多列，那么DISTINCT不起作用，它返回所有记录而没有任何Distinct查询的影响。我不知道我的代码有什么问题。请建议任何解决方案，并指导我在哪里出错。谢谢。

Code:

码：

@SuppressWarnings({ "unchecked" })
    @ApiMethod(name = "getDistinctFollow", httpMethod = HttpMethod.GET, path = "userendpoint/userName_fk3")
    public ObjectListContainer getDistinctFollower(
            @Named("followName") ArrayList<String> lstFollower,
            @Nullable @Named("cursor") String cursorString,
            @Nullable @Named("limit") Integer limit) {

        EntityManager mgr = null;
        Cursor cursor = null;
        List<Object[]> lstNames;
        List<String> lstString;
        ObjectListContainer object;
        try {
            mgr = getEntityManager();
            Query query = mgr.createQuery("select distinct f.uFullName,f.uUrl from UMaster f where f.uName in (:followName)");
            query.setParameter("followName", lstFollower);
            if (cursorString != null && cursorString != "") {
                cursor = Cursor.fromWebSafeString(cursorString);
                query.setHint(JPACursorHelper.CURSOR_HINT, cursor);
            }
            if (limit != null) {
                query.setFirstResult(0);
                query.setMaxResults(limit);
            }
            lstNames = (List<Object[]>) query.getResultList();
            cursor = JPACursorHelper.getCursor(lstNames);
            if (cursor != null)
                cursorString = cursor.toWebSafeString();

            lstString = new ArrayList<String>();

            for (Object obj[] : lstNames) {
                lstString.add(obj != null ? obj[0].toString() : null);
                lstString.add(obj != null ? obj[1].toString() : null);
            }
            object = new ObjectListContainer();
            object.setObjectsList(lstString);

        } finally {
            mgr.close();
        }
        return object;
    }

1 个解决方案

#1

Notice that DISTINCT only works with Indexed properties. Is any of uFullName or uName are not indexed, then the projection won't work.

请注意，DISTINCT仅适用于Indexed属性。 uFullName或uName中的任何一个都没有编入索引，那么投影将不起作用。