rsdb/prefer_join is a SAP Parameter attribute which is used to control set if JOIN shall be used for FOR ALL ENTRIES information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
This parameter is only intended for internal use and must not be changed
without explicit instruction from SAP.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.