[ORC-961] [C++] Expose metrics of the reader - ASF JIRA

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Details

Type: New Feature
Status: Resolved
Priority: Major
Resolution: Fixed
Affects Version/s: None
Fix Version/s: 1.9.0
Component/s: C++
Labels:
None

Description

It'd be useful for clients to collect metrics of the evaluation of the pushed down predicates. Currently SargsApplier keeps track of the selected RowGroups and evaluated RowGroups:
https://github.com/apache/orc/blob/40ee321b209cad1de73fc97b653636aa0fff28f6/c++/src/sargs/SargsApplier.hh#L79-L81

However, we are lack of an interface in RowReader to expose the metrics. Note that sargsApplier is a private member inside RowReaderImpl. We need an interface to expose it or only its stats.

It'd also be useful if we expose other metrics of a RowReader, e.g. acutal mem usage, time elapsed in decoding/decompression, time elapsed in evaluating SearchArguments.