In the case of rowets non-overlap and desc sorting, the logic of VCollectIterator::Level0Iterator::init_for_union will be followed. In this function, the row ref pos of the first level0 iterator is set to 0, and the row pos of other level0 iterators are all Set to -1.
But in the level1iterator, when rowets are non-overlapping and is ordering by desc, the list of rowset iterators will be reversed, causing the row ref pos of the first level0 iterator in the list to be -1, causing the block reader to think that the entire tablet has no data.
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