Query to get all unresolved issues of a specific workflow

[Megallion]

I have a listener with a function that when it executes, I need to get all the issues in the company's jira system that are unresolved and are using a specific workflow. This is a very large operation that is going to run since there will be a large number of issues.

I'm still new so I have a few questions.

How exactly should I go about doing something like this? Is it similar to a background re-index? What is the best way to query this kind of data?