stiga-huang ddef2cb9b1 IMPALA-376: add built-in functions for parsing JSON ...

This patch implements the same function as Hive UDF get_json_object.
We reuse RapidJson to parse the json string. In order to track the
memory used in RapidJson, we wrap FunctionContext into an allocator.

get_json_object accepts two parameters: a json string and a selector
(json path). We parse the json string into a Document tree and then
perform BFS according to the selector. For example, to process
    get_json_object('[{\"a\":1}, {\"a\":2}, {\"a\":3}]', '$[*].a'),
we first perform '$[*]' to extract all the items in the root array.
Then we get a queue consists of {a:1},{a:2},{a:3} and perform '.a'
selector on all values in the queue. The final results is 1,2,3 in the
queue. As there're multiple results, they should be encapsulated into
an array. The output results is a string of '[1,2,3]'.

More examples can be found in expr-test.cc.

Test:
* Add unit tests in expr-test
* Add e2e tests in exprs.test
* Add tests in test_alloc_fail.py to check handling of out of memory

Change-Id: I6a9d3598cb3beca0865a7edb094f3a5b602dbd2f
Reviewed-on: http://gerrit.cloudera.org:8080/10950
Reviewed-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>
Tested-by: Impala Public Jenkins <impala-public-jenkins@cloudera.com>

2018-09-29 11:59:03 +00:00

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