presto-main/src/main/java/io/prestosql/sql/planner/LocalExecutionPlanner.java/Visitor/createJoinSourcesLayout
class Visitor:        private Map<Symbol, Integer> createJoinSourcesLayout(Map<Symbol, Integer> lookupSourceLayout, Map<Symbol, Integer> probeSourceLayout)
        {
            ImmutableMap.Builder<Symbol, Integer> joinSourcesLayout = ImmutableMap.builder();
            joinSourcesLayout.putAll(lookupSourceLayout);
            for (Map.Entry<Symbol, Integer> probeLayoutEntry : probeSourceLayout.entrySet()) {
                joinSourcesLayout.put(probeLayoutEntry.getKey(), probeLayoutEntry.getValue() + lookupSourceLayout.size());
            }
            return joinSourcesLayout.build();
        }