The DocumentMK's VersionGCSupport code currently special-cases the Mongo persistence.
Needed are two optimized queries:
1) deletedOnce = true AND modifiedInSecs < lastModified
2) sdType in (....) AND sdMaxRevTimeInSecs < oldestRevTimeStamp
(where in the case of 2) the matching documents can be removed right away)
a) find out which of these system properties ought to be indexed properties
b) decide on whether we can implement this as generic interface extension of DocumentStore, or whether we add custom code for the RDBDocumentStore similary to what Mongo does
on b): I think we should aim for removing persistence-specific code from the kernel, so we probably should add extension interfaces for these.