[GitHub] [drill] paul-rogers opened a new pull request #1913: DRILL-6953: EVF-based version of the JSON reader

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paul-rogers opened a new pull request #1913: DRILL-6953: EVF-based version of the JSON reader
URL: https://github.com/apache/drill/pull/1913
 
 
   Reimplements the JSON reader on top of the EVF. Does not yet
   handle a provided schema. New JSON parser does not yet reflect
   any changes made to the "V1" JSON parser in the last year.
   Does not yet handle the Union and List-of-union types.
   Enabling those encountered many issues elsewhere in Drill.
   
   Provides more robust (but still limited) handling of JSON
   type ambigutities. Handles runs of nulls before the first
   non-null value (within the first batch.) Handles runs of
   empty arrays before the first non-empty array (again, within
   the first batch.) Handles the case where a null value turns out
   to be an object or array. Handles reasonable conversions between
   types.
   
   Handling ambiguities makes the new parser more complex than
   the "V1" version. The new one uses explict states for each
   kind of JSON object, where as the old one used implicit states
   expressed via if-statements, which can be a bit hard to follow
   as the states get more complex.
   
   The new "V2" JSON scan is controlled by a new option:
   store.json.enable_v2_reader, which is false by default in this
   PR.
   
   Adds a "projection type" to the column writer so that the
   JSON parser can receive a "hint" as to the expected type.
   The hint is from the form of the projected column: `a[0]`,
   `a.b` or just `a`.
   
   Reimplements a number of JSON tests to test both the original
   "V1" and the new "V2" versions of the JSON reader. Adds many
   new tests for the new features of the "V2" parser.

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