与SQL COUNT（*）聚合函数对应的CouchDB是什么？ [英] What is the CouchDB equivalent of the SQL COUNT(*) aggregate function?

问题描述

是的，我是一位进入CouchDb Map / Reduce世界的SQL骑师（sorta）。我想我已经用以下方法弄清楚了CouchDB数据集的COUNT（*）SQL聚合函数的等效方式：

Yep, I'm a SQL jockey (sorta) coming into the CouchDb Map/Reduce world. I thought I had figured out how the equivalent of the COUNT(*) SQL aggregator function for CouchDB datasets with the following:

Map：

function(doc) {
  emit(doc.name, doc);
}

减少：

function(keys, values, rereduce){
  return values.length;
}

我认为可行，返回的内容如下：

Which I thought worked, returning something like:

"super fun C"   2
"super fun D"   2
"super fun E"   2
"super fun F"   18

...但并非如此。当我添加一条记录时，此计数变化很大。有时，计数实际上减少了，这非常令人惊讶。难道我做错了什么？也许我不太了解最终一致性的概念？

... but not really. When I add a record, this count varies wildly. Sometimes the count actually decreases, which was very surprising. Am I doing something wrong? Maybe I don't fully understand the concept of eventual consistency?

推荐答案

看来，您的简化结果正在减少。也就是说，对于每个键，多次调用 reduce ，然后使用这些结果再次调用。您可以使用 reduce 函数来解决此问题，例如：

It looks like your reduce results are being re-reduced. That is, reduce is called more than once for each key and then called again with those results. You can handle that with a reduce function like this:

function(keys, values, rereduce) {
  if (rereduce) {
    return sum(values);
  } else {
    return values.length;
  }
}

或者，您可以更改 map 函数，以便该值始终是文档计数：

Alternatively, you can change the map function so that the values are always a count of documents:

// map
function(doc) {
  emit(doc.name, 1);
}

// reduce
function(keys, values, rereduce) {
  return sum(values);
}

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