Currently the only way to check rel node tree deep equality is transforming into String by RelOptUtil.toString(rel) with SqlExplainLevel.EXPPLAN_ATTRIBUTES, which is inefficient. One example is RexSubQuery. It has to do it this way, because the rel being reference by RexSubQuery is possibly not yet registered to VolcanoPlanner, and the digest equals checks the input RelNode by identity (not content). That is OK for RelSubset and HepRelVertex, if the RelNode is already registered in planner, but not for plain RelNode that is outside of planner. Due to this, we have to implement another set of deep equals logic in our system.