code: @Nullable
    private static DimFilter toLeafFilter(
        final PlannerContext plannerContext,
        final DruidQuerySignature querySignature,
        final RexNode rexNode
    )
    {
      if (rexNode.isAlwaysTrue()) {
        return Filtration.matchEverything();
      } else if (rexNode.isAlwaysFalse()) {
        return Filtration.matchNothing();
      }
  
      final DimFilter simpleFilter = toSimpleLeafFilter(
          plannerContext,
          querySignature,
          rexNode
      );
      return simpleFilter != null
             ? simpleFilter
             : toExpressionLeafFilter(plannerContext, querySignature.getRowSignature(), rexNode);
    }
docs: Translates "condition" to a Druid filter, assuming it does not contain any boolean expressions. Returns null
  if we cannot translate the condition.
  
  @param plannerContext planner context
  @param querySignature   row signature of the dataSource to be filtered
  @param rexNode        Calcite row expression