[jira] [Commented] (DRILL-2192) DrillScanRel should differentiate skip-all & scan-all & scan-some semantics while creating a GroupScan [umbrella]

["Parth Chandra (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/DRILL-2192?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14481493#comment-14481493 ] 

Parth Chandra commented on DRILL-2192:
--------------------------------------

Reassigning to Steven for the two remaining subtasks.

> DrillScanRel should differentiate skip-all & scan-all & scan-some semantics while creating a GroupScan [umbrella]
> -----------------------------------------------------------------------------------------------------------------
>
>                 Key: DRILL-2192
>                 URL: https://issues.apache.org/jira/browse/DRILL-2192
>             Project: Apache Drill
>          Issue Type: Improvement
>          Components: Query Planning & Optimization
>            Reporter: Hanifi Gunes
>            Assignee: Steven Phillips
>             Fix For: 1.0.0
>
>
> DrillScanRel passes a list of columns to be read into GroupScan. Currently the logic here is to scan all of the columns even if planner asks to skip them all. Skipping all of the columns is particularly beneficial for the case of count(star) that is translated to count(constant) where we just need row count but not the actual data.
> The idea is to distinguish three separate states depending on the output coming from planner as follows:
> | list of columns from planner | scan semantics |
> | null | scan-all |
> | empty list of columns | skip-all |
> | non-empty list of columns w/o star | scan-some |
> | list of columns with star | scan-all |
> As part this umbrella, we should make readers understand skip-all semantics.



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