Briefly, this error occurs when you try to use the `inner_hits` feature with a field that is used for collapsing results in Elasticsearch. The `inner_hits` feature is not compatible with the `collapse` feature. To resolve this issue, you can either remove the `inner_hits` feature from your query or remove the `collapse` feature. Alternatively, you can use a different field for collapsing that doesn’t require the use of `inner_hits`.
This guide will help you check for common problems that cause the log ” cannot expand `inner_hits` for collapse field ` ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: search.
Overview
Search refers to the searching of documents in an index or multiple indices. The simple search is just a GET API request to the _search endpoint. The search query can either be provided in query string or through a request body.
Examples
When looking for any documents in this index, if search parameters are not provided, every document is a hit and by default 10 hits will be returned.
GET my_documents/_search
A JSON object is returned in response to a search query. A 200 response code means the request was completed successfully.
{
"took" : 1,
"timed_out" : false,
"_shards" : {
"total" : 2,
"successful" : 2,
"failed" : 0
},
"hits" : {
"total" : 2,
"max_score" : 1.0,
"hits" : [
...
]
}
}Notes and good things to know
- Distributed search is challenging and every shard of the index needs to be searched for hits, and then those hits are combined into a single sorted list as a final result.
- There are two phases of search: the query phase and the fetch phase.
- In the query phase, the query is executed on each shard locally and top hits are returned to the coordinating node. The coordinating node merges the results and creates a global sorted list.
- In the fetch phase, the coordinating node brings the actual documents for those hit IDs and returns them to the requesting client.
- A coordinating node needs enough memory and CPU in order to handle the fetch phase.
Log Context
Log “cannot expand `inner_hits` for collapse field `” class name is CollapseBuilder.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :
if (fieldType.hasDocValues() == false) {
throw new SearchContextException(context; "cannot collapse on field `" + field + "` without `doc_values`");
}
if (fieldType.indexOptions() == IndexOptions.NONE && (innerHits != null && !innerHits.isEmpty())) {
throw new SearchContextException(context; "cannot expand `inner_hits` for collapse field `"
+ field + "`; " + "only indexed field can retrieve `inner_hits`");
} return new CollapseContext(field; fieldType; innerHits);
}
