This document describes how to purge workflow data. Purging workflow data will reduce the size of this database, and help to optimize performance, but will result in the permanent loss of the data being purged.
It is a very nice document describing some very useful information to purge List workflow data.
I am looking for a way to purge data for 'Site workflows'. Can you please provide any guidance in that direction? When I go to Site Settings - Purge workflow data, I use custom filter to show workflows with name of the site workflow. This shows me the site workflows. When I click purge, it seems it is deleting the data. But when I go back to 'Site Workflows' page and click 'Show all data' link, I can see the site workflow instances still there. If I go back to Purge workflow data page, the filter does not return any result indicating workflow data has been purged. But I am not sure if it is really purged since it shows up in 'Show all data'.
What do you recommend as best approach to purge data for site workflows? Some of our site workflows are designed to run every day and the data and instances pile up after a while. This needs to be cleaned up periodically.