使用ES聚合操作对字段(projectCode)去重：
GET bury-point-click/_search
{
"size": 0,
"aggs": {
"group_by_project": {
"cardinality": {
"field": "projectCode"
}
}
}
}执行报错：Fielddata is disabled on text fields by default. Set fielddata=true on [projectCode] in order to load fielddata in memory by uninverting the inverted index. Note that this can however use significant memory. Alternatively use a keyword field instead.
这是因为ES对排序，聚合这些操作用单独的数据结构(fielddata)缓存到内存里，Text字段数据会占用大量堆空间，需要单独开启。
开启代码如下：
PUT bury-point-click/_mapping/
{
"properties": {
"projectCode": {
"type": "text",
"fielddata": true
}
}
}再执行原来的语句，正常返回结果。
参考
https://www.elastic.co/guide/en/elasticsearch/reference/current/fielddata.html
《“ES聚合操作 – Fielddata is disabled on text fields by default”》 有 1 条评论
在es 5.3 以后，去重可以使用 Collapsing 字段折叠。在查询关键字加上keyword也可以实现聚合操作。
GET bury-point-click/_search/
{
"size": 0,
"aggs": {
"group_by_project": {
"terms": {
"field": "projectCode.keyword"
}
}
}
}