Unable to use partial string match when searching user in assignee field

Summary

Unable to use partial string match when searching user in assignee field.
This problem happened only in case that JIRA is configured to use DB for searching user instead of in-memory Lucene index.

Environment

The following property is added in <JIRA_HOME>/jira-config.properties.

jira.luceneUserSearcher = false

Steps to Reproduce

1. Add the following property to <JIRA_HOME>/jira-config.properties

   jira.luceneUserSearcher = false
   

2. Start JIRA
3. Create a user whose full name is "John Smith"
4. Give "Assignable User" permission to "John Smith" on an project
5. Open an issue in the project
6. Press Assign button
7. Type "Smith" in the assignee field

Expected Results

The user "John Smith" should be appears in the suggestion in the field.

Actual Results

The user "John Smith" didn't appear in the suggestion. (In case that we type "John", the user was shown in the suggestion.)

Notes

• The partial string matching worked on JIRA Server 7.1.2 even though the version still used DB for searching users
• When it comes to large JIRA instances, we still need to use jira.luceneUserSearcher = false option to avoid some problems like JRASERVER-65057
• Please see my internal comment for the additional information

Workaround

Use in-memory Lucene index for searching users. (Disable jira.luceneUserSearcher = false option)