presto-main/src/main/java/io/prestosql/operator/PartitionedOutputOperator.java/PagePartitioner/getPartitionFunctionArguments
class PagePartitioner:        private Page getPartitionFunctionArguments(Page page)
        {
            Block[] blocks = new Block[partitionChannels.size()];
            for (int i = 0; i < blocks.length; i++) {
                Optional<Block> partitionConstant = partitionConstants.get(i);
                if (partitionConstant.isPresent()) {
                    blocks[i] = new RunLengthEncodedBlock(partitionConstant.get(), page.getPositionCount());
                }
                else {
                    blocks[i] = page.getBlock(partitionChannels.get(i));
                }
            }
            return new Page(page.getPositionCount(), blocks);
        }