[rust][datafusion] optimize count(*) queries on parquet sources

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-8902

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)

[alamb]

wrong repo