presto-main/src/main/java/io/prestosql/type/TypeUtils.java/TypeUtils/getHashPage
class TypeUtils:    public static Page getHashPage(Page page, List<? extends Type> types, List<Integer> hashChannels)
    {
        ImmutableList.Builder<Type> hashTypes = ImmutableList.builder();
        Block[] hashBlocks = new Block[hashChannels.size()];
        int hashBlockIndex = 0;

        for (int channel : hashChannels) {
            hashTypes.add(types.get(channel));
            hashBlocks[hashBlockIndex++] = page.getBlock(channel);
        }
        return page.appendColumn(getHashBlock(hashTypes.build(), hashBlocks));
    }