Bucket_selector aggregation and size. optimizations - elasticsearch

Question

I have question about bucket_selector aggregation.
(Environment tested: ES6.8 and ES7 basic on centos7)
In my use case I need to drop documents if there are dupes by selected property. Index is not big about 2mln records.
Query to find those records looks like this:
GET index_id1/_search
{
"size": 0,
"aggs": {
"byNested": {
"nested": {
"path": "nestedObjects"
},
"aggs": {
"sameIds": {
"terms": {
"script": {
"lang": "painless",
"source": "return doc['nestedObjects.id'].value"
},
"size": 1000
},
"aggs": {
"byId": {
"reverse_nested": {}
},
"byId_bucket_filter": {
"bucket_selector": {
"buckets_path": {
"totalCount": "byId._count"
},
"script": {
"source": "params.totalCount > 1"
}
}
}
}
}
}
}
}
}
I get the buckets back. But to relax the query and the load. I do it by size: 1000. So, next query issued to get more dupes until zero is back.
The problem is however - too small amount of dupes. I checked the result of the query by setting size: 2000000:
GET index_id1/_search
{
"size": 0,
"aggs": {
"byNested": {
"nested": {
"path": "nestedObjects"
},
"aggs": {
"sameIds": {
"terms": {
"script": {
"lang": "painless",
"source": "return doc['nestedObjects.id'].value"
},
"size": 2000000 <-- too big
},
"aggs": {
"byId": {
"reverse_nested": {}
},
"byId_bucket_filter": {
"bucket_selector": {
"buckets_path": {
"totalCount": "byId._count"
},
"script": {
"source": "params.totalCount > 1"
}
}
}
}
}
}
}
}
}
As I understand first step is: it actually creates the buckets as stated in the query and then bucket_selector filters only what i need. And that's why i see this kind of behavior. In order to get all the buckets I have to adjust "search.max_buckets" to 2000000.
Converted to query with composite aggregation:
GET index_id1/_search
{
"aggs": {
"byNested": {
"nested": {
"path": "nestedObjects"
},
"aggs": {
"compositeAgg": {
"composite": {
"after": {
"termsAgg": "03f10a7d-0162-4409-8647-c643274d6727"
},
"size": 1000,
"sources": [
{
"termsAgg": {
"terms": {
"script": {
"lang": "painless",
"source": "return doc['nestedObjects.id'].value"
}
}
}
}
]
},
"aggs": {
"byId": {
"reverse_nested": {}
},
"byId_bucket_filter": {
"bucket_selector": {
"script": {
"source": "params.totalCount > 1"
},
"buckets_path": {
"totalCount": "byId._count"
}
}
}
}
}
}
}
},
"size": 0
}
As I understand it does the same thing except that I need to make 2000 calls (size: 1000 each) to go over the whole index.
Is composite agg caches the results or why this is better?
Maybe there is a better approach in this case?