[jira] [Created] (DRILL-6791) Merge scan projection framework into master

["Paul Rogers (JIRA)" <ji...@apache.org>]

Paul Rogers created DRILL-6791:
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             Summary: Merge scan projection framework into master
                 Key: DRILL-6791
                 URL: https://issues.apache.org/jira/browse/DRILL-6791
             Project: Apache Drill
          Issue Type: Improvement
    Affects Versions: 1.15.0
            Reporter: Paul Rogers
            Assignee: Paul Rogers


Merge the next set of "result set loader" code into master via a PR. This one covers the "schema projection" mechanism which:

* Handles none (SELECT COUNT(*)), some (SELECT a, b, x) and all (SELECT *) projection.
* Handles null columns (for projection a column "x" that does not exist in the base table.)
* Handle file metadata (AKA implicit columns)
* Handle schema persistence: the need to reuse the same vectors across different scanners
* Provides a framework for consuming externally-supplied metadata
* Baring "real" metadata, obtains metadata hints from previous batches and from the projection list (a.b implies that "a" is a map, c[0] implies that "c" is an array, etc.)




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