presto-main/src/main/java/io/prestosql/sql/planner/optimizations/IndexJoinOptimizer.java/IndexKeyTracer/visitIndexJoin
class IndexKeyTracer:            @Override
            public Map<Symbol, Symbol> visitIndexJoin(IndexJoinNode node, Set<Symbol> lookupSymbols)
            {
                Set<Symbol> probeLookupSymbols = lookupSymbols.stream()
                        .filter(node.getProbeSource().getOutputSymbols()::contains)
                        .collect(toImmutableSet());
                checkState(!probeLookupSymbols.isEmpty(), "No lookup symbols were able to pass through the index join probe source");
                return node.getProbeSource().accept(this, probeLookupSymbols);
            }