Todd Lipcon created KUDU-2844:
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Summary: Avoid copying strings from dictionary or plain-encoded
blocks
Key: KUDU-2844
URL: https://issues.apache.org/jira/browse/KUDU-2844
Project: Kudu
Issue Type: Improvement
Components: cfile, perf
Reporter: Todd Lipcon
When scanning a plain or dictionary-encoded binary column, we currently loop
over each entry and copy the string into the destination RowBlock's arena. In
TPCH Q1, the scanner threads use a significant percentage of CPU doing this
copying, and it also increases CPU cache footprint which likely decreases
performance in downstream operations like predicate evaluation, merging, result
serialization, etc.
Instead of doing this, we could "attach" the dictionary block (with
ref-counting) to the RowBlock and refer directly to the dictionary entry from
the RowBlock. When the RowBlock eventually is reset, we can drop the reference.
This should be safe because we never mutate indirect data in-place.
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