Versions of IDM prior to 6.0 have this index defined on the "relationships" table:
In 6.0.0, the openidm repo initialization script redefines this index like so:
6.0.0 also provides a "hybridize" update script which alters the relationships table and adds two more indexes. However, it does not update the definition for this index. As a result, the relationships table does not get indexed for proper usage. Anyone upgrading to 6.0 (or beyond) needs to make sure to fix this index.