Elasticsearch does not find partial matches in project names

Summary

Zendesk: https://gitlab.zendesk.com/agent/tickets/36081

Built-in database search will find results for partial matches, while Elasticsearch only finds full/specific keywords

Steps to reproduce

Without Elasticsearch enabled, search for a project, such as 'gitlab' - You should find 'gitlab-ce' and 'gitlab-ee' in results, plus others. Enable Elasticsearch, update indexes, and search for 'gitlab' again. Notice that results such as 'gitlab-ce' and 'gitlab-ee' no longer appear.

Expected behavior

Elasticsearch searches should return partial matches.

Changed title: Elasticsearch does not find partial matches → Elasticsearch does not find partial matches in project names

I changed title of the issue. And I did some tests. Actually it finds parts if there is "-" but not "_"

Also that's not a bug. ES search does not copy database search behaviour and it should/can behave differently.

I think that we should improve search to be able to search partial matches where underscore (_) is used. Also we should consider using ngram tokenizer to match not split parts like "gitlab" in "gitlabce". Ngram pretty expensive because it requires more disk space but I think for project names it's OK.

Removed ~91797 label

Milestone changed to %8.12

Assignee removed

I removed assignee to not prevent someone to pick this task up. Anyway I have put it to my TODO list

Reassigned to @vsizov

mentioned in merge request !726 (merged)

Fix https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/726

Status changed to closed by merge request !726 (merged)

Mentioned in commit 45402b12