do not read seq column when reading a compacted rowset #10344
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SEQ_COL is used on tables with unique key to order data in one transaction(rowset), when there is only one rowset and the rowset is compacted, rows in the rowset is sorted and rows with same keys are resolved by compaction, so a scanner sets direct_mode to optimize read iterator to avoid sorting and aggregating, and iterators does not need SEQ_COL. However, init_return_columns adds SEQ_COL to return_columns, which is passed to SegmentIterator. Then segment Iterator would be called via get_next with a block without SEQ_COL, segment iterator creates columns included in return_columns but not in the block. SEQ_COL is nullable, segment Iterator does not handle it, so a core dump happen. Actually, in the above case, segment iterator does not need to read SEQ_COL. When SEQ_COL is really needed, iterators creates SEQ_COL column in block, so segment Iterator does not need do create SEQ_COL at all.
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Proposed changes
Issue Number: close #10337
Problem Summary:
SEQ_COL is used on tables with unique key to order data in one transaction(rowset), when there is only one rowset and the rowset is compacted, rows in the rowset is sorted and rows with same keys are resolved by compaction, so a scanner sets direct_mode to optimize read iterator to avoid sorting and aggregating, and iterators does not need SEQ_COL. However, init_return_columns adds SEQ_COL to return_columns, which is passed to SegmentIterator. Then segment Iterator would be called via get_next with a block without SEQ_COL, segment iterator creates columns included in return_columns but not in the block. SEQ_COL is nullable, segment Iterator does not handle it, so a core dump happen.
Actually, in the above case, segment iterator does not need to read SEQ_COL. When SEQ_COL is really needed, iterators creates SEQ_COL column in block, so segment Iterator does not need do create SEQ_COL at all.
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