flashzxi

At this stage, expression evaluation is based on the volcano model, and the evaluate function is called to the root node of the expression tree, and the tree will recursively...

GH-43124: [C++] Initialize offset vector head as 0 after memory allocated in grouper.cc

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## bug description In grouper.cc:780, memory is allocated for offset vector of varlen column, however the vector is initialized in `encoder_.DecodeFixedLengthBuffers`, which will never be called when `num_groups==0`(see line 786)....

[C++] Allocate memory of a random size because the passed value is an uninitialized int32_t.

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### Describe the bug, including details regarding any error messages, version, and platform. In grouper.cc, allocated a random uint32_t size memory because of using uninitialized offset vector when `group_nums ==...