presto-main/src/main/java/io/prestosql/sql/planner/optimizations/IndexJoinOptimizer.java/IndexKeyTracer/visitIndexSource
class IndexKeyTracer:            @Override
            public Map<Symbol, Symbol> visitIndexSource(IndexSourceNode node, Set<Symbol> lookupSymbols)
            {
                checkState(node.getLookupSymbols().equals(lookupSymbols), "lookupSymbols must be the same as IndexSource lookup symbols");
                return lookupSymbols.stream().collect(toImmutableMap(identity(), identity()));
            }