Alex Gaynor created ARROW-8902:
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Summary: [rust][datafusion] optimize count(*) queries on parquet
sources
Key: ARROW-8902
URL: https://issues.apache.org/jira/browse/ARROW-8902
Project: Apache Arrow
Issue Type: Bug
Components: Rust - DataFusion
Reporter: Alex Gaynor
Currently, as far as I can tell, when you perform a `select count(*) from
dataset` in datafusion against a parquet dataset, the way this is implemented
is by doing a scan on column 0, and counting up all of the rows (specifically I
think it counts the # of rows in each batch).
However, for the specific case of just counting _everythign_ in a parquet file,
you can just read the rowcount from the footer metadata, so it's O(1) instead
of O(n)
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