How to find issues that at some point has been assigned to you?

Check out JIRA Toolkit plugin - Participants custom field

https://studio.plugins.atlassian.com/wiki/display/JTOOL/JIRA+Toolkit+Plugin

This field allows you to easily track issues that you've "participated in". These are defined to be any issues you've commented on, raised or are the current assignee. See also the [JIRA Extended Participants] plugin.

This is meanwhile possible by means of the JIRA Query Language (JQL) operator 'WAS', which has been introduced in JIRA 4.3 and extended in JIRA 4.4 to cover assignees and reporters as well, for example:

project = "Angry Nerds" and (assignee was 'johnsmith' or reporter was 'johnsmith')

General-purpose query for whichever 'current user':

assignee was currentUser()

This filter can be conveniently shared & anybody can put it on their dashboard, etc and it will return results specific to them.. Not supported on all old JIRA versions though.

This was my most-requested JIRA feature ever.