Cassandra的计数器类型的总和 [英] aggregate sum on Counter type of Cassandra

问题描述

我使用Cassandra 2.2.0，我已阅读使用计数器类型，但它不提供太多的细节。

I'm using Cassandra 2.2.0, and I've read using Counter type but it doesn't provide much detail.

有一种方法聚集在计数器类型列为Cassandra，类似于以下？

Is there a way to aggregate on Counter type column for Cassandra, similar to the following?

SELECT sum(my_counter_column) FROM my_table ;

上述查询会导致此错误：

The above query results in this error:

InvalidRequest: code=2200 [Invalid query] message="Invalid call to
function sum, none of its type signatures match (known type
signatures: system.sum : (tinyint) -> tinyint, system.sum : (smallint)
-> smallint, system.sum : (int) -> int, system.sum : (bigint) -> bigint, 
system.sum : (float) -> float, system.sum : (double) ->
double, system.sum : (decimal) -> decimal, system.sum : (varint) ->
varint)"

我知道我可以获取所有数据聚合在客户端，但我只是想知道是否可以在Cassandra中完成。非常感谢。

I know I can fetch all data and then do aggregation in the client, but I'm just wondering if it can be done within Cassandra. Thanks a lot.

推荐答案

当系统 sum（）函数。您应该可以为它输入一个jira票这里为2.2.x和3.x，以便它可以修复。

Looks like an oversight when the system sum() function was implemented. You should probably enter a jira ticket for it here for 2.2.x and 3.x so that it can be fixed.

同时，你可以解决问题定义您自己的用户定义的聚合函数（在Cassandra 2.2和更高版本中），如下所示：

In the meantime you can work around the problem by defining your own user defined aggregate function (in Cassandra 2.2 and later) like this:

CREATE FUNCTION agg_counter ( state bigint, val counter )
  CALLED ON NULL INPUT
  RETURNS bigint
  LANGUAGE java
  AS '
      if (val != null)
          state = state + val;
      return state;
  ';

CREATE AGGREGATE sum_counter ( counter )
  SFUNC agg_counter
  STYPE bigint
  INITCOND 0;

然后你会这样使用：

SELECT sum_counter(countercol) FROM table WHERE partition=1;

我试过在3.0.0-beta2和它的工作原理。它也应该在2.2中工作。

I tried that in 3.0.0-beta2 and it works. It should also work in 2.2.

在你尝试之前，请记住在cassandra.yaml中设置 enable_user_defined_functions：true 创建函数。

Just remember to set enable_user_defined_functions: true in cassandra.yaml before you try to create the function.

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