There should be a way for setting a substitute for a given assignee (user)

Each company using Jira faces the problem from time to time: a user goes to vacation or goes sick, so some other person should process all issues where the absent user is assignee. Currently the only option is to find and process such issues by hand, re-assign them if necessary, change other custom fields, where the absent user is mentioned. It's not convenient.

I suggest Jira provides a way to assign a substitute for any user. For example, in the Users list there should be a field Current substitute, and if it is filled in, the given user is used anywhere in the algorithms (including plugins' in/out parameters) instead of the original one.