Ben Kietzman created ARROW-11647:
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Summary: [C++][Compute] CastFromNull does not use preallocated
buffers
Key: ARROW-11647
URL: https://issues.apache.org/jira/browse/ARROW-11647
Project: Apache Arrow
Issue Type: Improvement
Components: C++
Reporter: Ben Kietzman
Assignee: Ben Kietzman
Fix For: 4.0.0
When casting from null, currently new buffers are allocated for every batch of
the computation. This is wasteful as for simple types data buffers are
preallocated and the null bitmap is handled separately; CastFromNull need do no
work at all (unless we decide to explicitly zero the data buffer). For
varlength out types the offsets buffer is preallocated and should be zeroed,
for struct types preallocation is not implemented (but should be as simple as
preallocating each child array).
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