# What's the difference between fielddata\_fields and docvalue\_fields in inner hits?
**URL:** https://discuss.elastic.co/t/whats-the-difference-between-fielddata-fields-and-docvalue-fields-in-inner-hits/123304
**Category:** Elasticsearch
**Created:** [March 9, 2018, 4:39pm UTC](https://discuss.elastic.co/t/whats-the-difference-between-fielddata-fields-and-docvalue-fields-in-inner-hits/123304 "2018-03-09T16:39:57Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@BLZB0B](https://discuss.elastic.co/u/BLZB0B)
#### Post date: [March 9, 2018, 4:39pm UTC](https://discuss.elastic.co/t/whats-the-difference-between-fielddata-fields-and-docvalue-fields-in-inner-hits/123304/1 "2018-03-09T16:39:58Z")
Hi,
we've noticed a perfomance hit on our inner hits, doing a bit of digging, we worked out that this was being caused by not setting \_source:false on _some_ of our inner hits.
the following document
[https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-inner-hits.html#nested-inner-hits-source](https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-inner-hits.html#nested-inner-hits-source)
recommends settting source to false so that it doesn't need to traverse back and forth the parent document for each inner hit and to return the required values using docvalue\_fields.
on some of the other inner hits that have previously been "optimised", we're already using source: false but instead of docvalue\_fields, we're using fielddata\_fields thus:
```auto
"inner_hits":{
"size":100,
"_source": false,
"fielddata_fields":[
"doc.fq.path.to.innerhit.field",
"doc.fq.path.to.innerhit.field.keyword"
]
}
```
both _seem_ to perform similarly, both return the same results (albiet with a slighly different result "shape").
is there a preference of one over the other? I'm sure we're using fielddata\_fields after talking to @mvg months ago about nested nested inner hits.
the above tests were done using postman, but in "real" code, we use the NEST C# library to build the queries - not sure if there's a limit in there for one over the other..
Any guidance would be appreciated.
Regards
Phil
---
### Author:  [@system](https://discuss.elastic.co/u/system)
#### Post date: [April 6, 2018, 4:40pm UTC](https://discuss.elastic.co/t/whats-the-difference-between-fielddata-fields-and-docvalue-fields-in-inner-hits/123304/2 "2018-04-06T16:40:04Z")
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