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From Paul Rogers <prog...@mapr.com>
Subject JSON reader enhancement
Date Fri, 17 Nov 2017 20:39:29 GMT
Hi All,

I’d like to propose a minor enhancement to the JSON reader to better handle non-relational
JSON structures. (See DRILL-5974 [1].)

As background, Drill handles simple tuples:

{a: 10, b: “fred”}

Drill also handles arrays:

{name: “fred”, hobbies: [“bowling”, “golf”]}

Drill even handles arrays of tuples:

{name: “fred”, orders: [
  {id: 1001, amount: 12.34},
  {id: 1002, amount: 56.78}]}

The above are termed "relational" because there is a straightforward mapping to/from tables
into the above JSON structures.

Things get interesting with non-relational types, such as 2-D arrays:

{id: 4, shape: “square”, points: [[0, 0], [0, 5], [5, 0], [5, 5]]}

Drill has two solutions:

* Turn on the experimental list and union support.
* Enable all-text mode to read all fields as JSON text.

Here, I’d like to propose a middle ground:

* Read fields with relational types into vectors.
* Read non-relational fields using text mode.

Thus, the first three examples would all result in the JSON data parsed into Drill vectors.
But, the fourth, non-relational example would produce a row that looks like this:

id, shape, points
4, “shape”, “[[0, 0], [0, 5], [5, 0], [5, 5]]”

Although Drill can’t parse the 2-D array, Drill will pass the array along to the client,
which can use its favorite JSON parser to parse the array and do something useful (like draw
the square in this case.)

In particular, the proposal:

* Apply this change only to the revised “batch size aware” JSON reader.
* Use the above parsing model by default.
* Use the experimental list-and-union support if the existing `exec.enable_union_type` system/session
option is set.

Existing queries should “just work.” In fact, now JSON with non-relational types will
work “out-of-the-box” without all-text mode or the experimental types.


- Paul

[1] https://issues.apache.org/jira/browse/DRILL-5974

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