[jira] [Created] (SPARK-25894) Include a count of the number of physical columns read for a columnar data source in the metadata of FileSourceScanExec

["Michael Allman (JIRA)" <ji...@apache.org>]

Michael Allman created SPARK-25894:
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             Summary: Include a count of the number of physical columns read for a columnar data source in the metadata of FileSourceScanExec
                 Key: SPARK-25894
                 URL: https://issues.apache.org/jira/browse/SPARK-25894
             Project: Spark
          Issue Type: Improvement
          Components: SQL
    Affects Versions: 2.4.0
            Reporter: Michael Allman


Knowing the number of physical columns Spark will read from a columnar file format (such as Parquet) is extremely helpful (if not critical) in validating an assumption about that number of columns based on a given query and schema pruning functionality. For example, take a {{contacts}} table with a {{name}} struct like {{name.first, name.last}}. Without schema pruning the following query reads both columns in the {{name}} struct:

{{select name.first from contacts}}

With schema pruning, the same query reads only the {{name.first}} column.

This issue (and related PR) proposes an additional metadata field for {{FileSourceScanExec}} which identifies the number of columns Spark will read from that file source. This metadata will be printed as part of a physical plan explanation.



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