commit 66e8dd9f05ea068e27b272ed1b787004213c29e6
author Paul Rogers <par0328@yahoo.com> Sun Jun 23 22:10:54 2019 -0700
committer Arina Ielchiieva <arina.yelchiyeva@gmail.com> Mon Jul 08 13:42:53 2019 +0300
tree 45a3c9b2f5b15f368ac2f1f75db73ee8eb3550db
parent 1522dd0e016459a03436ade38134bd98e59f7e57

DRILL-7306: Disable schema-only batch for new scan framework

The EVF framework is set up to return a "fast schema" empty batch
with only schema as its first batch because, when the code was
written, it seemed that's how we wanted operators to work. However,
DRILL-7305 notes that many operators cannot handle empty batches.

Since the empty-batch bugs show that Drill does not, in fact,
provide a "fast schema" batch, this ticket asks to disable the
feature in the new scan framework. The feature is disabled with
a config option; it can be re-enabled if ever it is needed.

SQL differentiates between two subtle cases, and both are
supported by this change.

1. Empty results: the query found a schema, but no rows
   are returned. If no reader returns any rows, but at
   least one reader provides a schema, then the scan
   returns an empty batch with the schema.
2. Null results: the query found no schema or rows. No
   schema is returned. If no reader returns rows or
   schema, then the scan returns no batch: it instead
   immediately returns a DONE status.

For CSV, an empty file with headers returns the null result set
(because we don't know the schema.) An empty CSV file without headers
returns an empty result set because we do know the schema: it will
always be the columns array.

Old tests validate the original schema-batch mode, new tests
added to validate the no-schema-batch mode.