fix(nimble): Fix null population when abandoning the dictionary - #18628
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fix(nimble): Fix null population when abandoning the dictionary#18628HuamengJiang wants to merge 1 commit into
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Summary: A selective Nimble read of a string column returned NULL for rows that hold a real string. Reproducing it needs a filter pushed down onto the string column and dictionary preservation enabled. With both on, one 5.1M-row file yielded 4 wrong rows at a batch size of 100 and 7 at a batch size of 1000; the same scan without pushdown was correct. Only the null flags were wrong. The values, the output row count, and the surviving row set all matched the reference read. The dictionary path reads its prefix with the filter suppressed, so `filterDictionaryIndices` compacts the output afterwards down to the passing rows. It cleared the result null bitmap only up to the new, smaller `numValues_`. The vacated tail kept the null bits of the pre-compaction layout. If the read then reaches a chunk that is not dictionary-encoded, it abandons the dictionary and a flat continuation appends over that same tail. That continuation stores each value with `addValue`, which writes the value but never the null bit, and the buffer-wide clear in `prepareNulls` is skipped once the prefix has reported a null. Nothing else clears the leftover bits, so they surface as nulls. Both compaction branches now clear the whole range they compacted. Differential Revision: D116928592
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Summary:
A selective Nimble read of a string column returned NULL for rows that hold a real string. Reproducing it needs a filter pushed down onto the string column and dictionary preservation enabled. With both on, one 5.1M-row file yielded 4 wrong rows at a batch size of 100 and 7 at a batch size of 1000; the same scan without pushdown was correct. Only the null flags were wrong. The values, the output row count, and the surviving row set all matched the reference read.
The dictionary path reads its prefix with the filter suppressed, so
filterDictionaryIndicescompacts the output afterwards down to the passing rows. It cleared the result null bitmap only up to the new, smallernumValues_. The vacated tail kept the null bits of the pre-compaction layout. If the read then reaches a chunk that is not dictionary-encoded, it abandons the dictionary and a flat continuation appends over that same tail. That continuation stores each value withaddValue, which writes the value but never the null bit, and the buffer-wide clear inprepareNullsis skipped once the prefix has reported a null. Nothing else clears the leftover bits, so they surface as nulls. Both compaction branches now clear the whole range they compacted.Differential Revision: D116928592