按计数有效地聚合多个列

Question

给出一个表

Table
col_1    col_2    col_3    ...
1        2        4
4        2        4
4        1        2

我可以用

计算单个列

Table.objects.filter(key=int).values('col_1').annotate(Count('col_1'))

并获得该列的预期结果：

[{col_1: 4, col_1__count: 2}, {col_1: 1, col_1__count: 1}]

如何有效地（一个数据库往返）为多个列执行此操作以实现以下输出？

[{col_1: 4, col_1__count: 2}, {col_1: 1, col_1__count: 1}, 
 {col_2: 2, col_2__count: 2}, {col_2: 1, col_2__count: 1}, 
 {col_3: 4, col_3__count: 2}, {col_3: 2, col_3__count: 1},
 ...                                                      ]