# Understanding performance impact of parent/child mapping

**URL:** <https://discuss.elastic.co/t/understanding-performance-impact-of-parent-child-mapping/14606>\
**Category:** Elasticsearch\
**Created:** [November 26, 2013, 9:54pm UTC](https://discuss.elastic.co/t/understanding-performance-impact-of-parent-child-mapping/14606 "2013-11-26T21:54:00Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Emilie\_Lavigne](https://avatars.discourse-cdn.com/v4/letter/e/91b2a8/32.png) [@Emilie\_Lavigne](https://discuss.elastic.co/u/Emilie_Lavigne)\
**Post date:** [November 26, 2013, 9:54pm UTC](https://discuss.elastic.co/t/understanding-performance-impact-of-parent-child-mapping/14606/1 "2013-11-26T21:54:00Z")

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I am investigating whether the parent/child option is viable for our use  
case. I would like a few clarifications on how the id cache is populated.

- What gets loaded into the \_id cache? All document \_ids or only parent  
\_ids?
- Are child -\> parent mappings also loaded into the cache?
- If so, if a child defines a non-existing parent at index time, will  
that child-parent mapping still get loaded into the cache?

To be clearer, we have about 2 trillion documents indexed across 4 nodes,  
with a total of 400GB of RAM dedicated to ElasticSearch. In our use case,  
only about 10% of documents will likely have a parent to point to. Most  
will be orphans. I am trying to understand whether there is a way to  
prevent orphan documents (ie: documents that would point to a non-existing  
parent such as "NA") from having an impact on the heap memory.

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**Author:** ![system](https://us1.discourse-cdn.com/elastic/original/3X/1/a/1ac57faf039f6b580b3f104ef42a2a89e41014de.png) [@system](https://discuss.elastic.co/u/system)\
**Post date:** [July 6, 2017, 2:04am UTC](https://discuss.elastic.co/t/understanding-performance-impact-of-parent-child-mapping/14606/2 "2017-07-06T02:04:42Z")

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