最近入坑了，ElastaticSearch 计算(count)所有的个数，按某一个字段出现多次计算一次，所有有两种解决方案：
1.cardinality(重复字段)
如果要求容错率较低，可以用cardinality（2.X,其他也类似，40000之内能够基本准确，且不管查询的文档量是多少，即便是百万级也能够保证错误率在5%以下）。官方的介绍
This example will ensure that fields with 100 or fewer distinct values will be extremely accurate. Although not guaranteed by the algorithm, if a cardinality is under the threshold, it is almost always 100% accurate. Cardinalities above this will begin to trade accuracy for memory savings, and a little error will creep into the metric.
For a given threshold, the HLL data-structure will use about precision_threshold * 8 bytes of memory. So you must balance how much memory you are willing to sacri‐ fice for additional accuracy.
Practically speaking, a threshold of 100 maintains an error under 5% even when counting millions of unique values.
2.terms(重复字段)
在此方法下记得terms().field()是计算10个，所以需要记得在后面加上.size(0),我就是应为忘记了，所以计算了很多值都是10以及10以下．第一次可以有错，后面一定要引以为鉴．当然，计算个数时也变成了getBuckets().size()，其中我们要记得需要判断detBuckets().get(0).getDocCount()是否为0，为0时不统计．数据精确但是耗时需要多次校验．
如果有什么更好的方法，希望大家留言，让大家都试试．