presto-main/src/main/java/io/prestosql/operator/TableWriterOperator.java/TableWriterOperator/createFragmentsPage
class TableWriterOperator:    private Page createFragmentsPage()
    {
        Collection<Slice> fragments = getFutureValue(finishFuture);
        committed = true;
        updateWrittenBytes();

        // output page will only be constructed once,
        // so a new PageBuilder is constructed (instead of using PageBuilder.reset)
        PageBuilder page = new PageBuilder(fragments.size() + 1, ImmutableList.of(types.get(ROW_COUNT_CHANNEL), types.get(FRAGMENT_CHANNEL)));
        BlockBuilder rowsBuilder = page.getBlockBuilder(0);
        BlockBuilder fragmentBuilder = page.getBlockBuilder(1);

        // write row count
        page.declarePosition();
        BIGINT.writeLong(rowsBuilder, rowCount);
        fragmentBuilder.appendNull();

        // write fragments
        for (Slice fragment : fragments) {
            page.declarePosition();
            rowsBuilder.appendNull();
            VARBINARY.writeSlice(fragmentBuilder, fragment);
        }

        return page.build();
    }