presto-main/src/main/java/io/prestosql/operator/RowNumberOperator.java/RowNumberOperator/createRowNumberBlock
class RowNumberOperator:    private Block createRowNumberBlock()
    {
        BlockBuilder rowNumberBlock = BIGINT.createFixedSizeBlockBuilder(inputPage.getPositionCount());
        for (int currentPosition = 0; currentPosition < inputPage.getPositionCount(); currentPosition++) {
            long partitionId = getPartitionId(currentPosition);
            long nextRowCount = partitionRowCount.get(partitionId) + 1;
            BIGINT.writeLong(rowNumberBlock, nextRowCount);
            partitionRowCount.set(partitionId, nextRowCount);
        }
        return rowNumberBlock.build();
    }