presto-iceberg/src/main/java/io/prestosql/plugin/iceberg/IcebergPageSink.java/PagePartitioner/partitionPage
class PagePartitioner:        public int[] partitionPage(Page page)
        {
            Block[] blocks = new Block[columns.size()];
            for (int i = 0; i < columns.size(); i++) {
                PartitionColumn column = columns.get(i);
                Block block = page.getBlock(column.getSourceChannel());
                blocks[i] = column.getBlockTransform().apply(block);
            }
            Page transformed = new Page(page.getPositionCount(), blocks);

            return pageIndexer.indexPage(transformed);
        }