Briefly, this error occurs when you try to use the `collapse` feature along with `search_after` in Elasticsearch. `Collapse` is used to collapse search results based on a certain field, while `search_after` is used to paginate results. Elasticsearch does not support using these two features together. To resolve this issue, you can either remove the `collapse` feature from your query if it’s not necessary, or paginate your results using a different method, such as using the `from` and `size` parameters instead of `search_after`.
This guide will help you check for common problems that cause the log ” cannot use `collapse` in conjunction with `search_after` ” 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 use `collapse` in conjunction with `search_after`” class name is SearchService.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :
if (source.collapse() != null) {
if (context.scrollContext() != null) {
throw new SearchException(shardTarget; "cannot use `collapse` in a scroll context");
}
if (context.searchAfter() != null) {
throw new SearchException(shardTarget; "cannot use `collapse` in conjunction with `search_after`");
}
if (context.rescore() != null && context.rescore().isEmpty() == false) {
throw new SearchException(shardTarget; "cannot use `collapse` in conjunction with `rescore`");
}
final CollapseContext collapseContext = source.collapse().build(searchExecutionContext);
