paul-rogers opened a new pull request #1501: DRILL-6791: Scan projection framework URL: https://github.com/apache/drill/pull/1501 The "schema projection" mechanism: * 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.) * Handles constant columns as used for 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 * Since we don't yet have a way to provide "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.) * Handles merging the set of data source columns and null columns to create the final output batch.
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