[jira] [Reopened] (ARROW-8902) [rust][datafusion] optimize count(*) queries on parquet sources

["Andrew Lamb (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/ARROW-8902?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Andrew Lamb reopened ARROW-8902:
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> [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: Improvement
>          Components: Rust, Rust - DataFusion
>            Reporter: Alex Gaynor
>            Priority: Minor
>
> 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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