presto-main/src/main/java/io/prestosql/sql/planner/plan/ExchangeNode.java/ExchangeNode/mergingExchange
class ExchangeNode:    public static ExchangeNode mergingExchange(PlanNodeId id, Scope scope, PlanNode child, OrderingScheme orderingScheme)
    {
        PartitioningHandle partitioningHandle = scope == LOCAL ? FIXED_PASSTHROUGH_DISTRIBUTION : SINGLE_DISTRIBUTION;
        return new ExchangeNode(
                id,
                Type.GATHER,
                scope,
                new PartitioningScheme(Partitioning.create(partitioningHandle, ImmutableList.of()), child.getOutputSymbols()),
                ImmutableList.of(child),
                ImmutableList.of(child.getOutputSymbols()),
                Optional.of(orderingScheme));
    }