[GitHub] [calcite] XuQianJin-Stars opened a new pull request #1086: [CALCITE-2892]Add the JSON_KEYS function

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XuQianJin-Stars opened a new pull request #1086: [CALCITE-2892]Add the JSON_KEYS function
URL: https://github.com/apache/calcite/pull/1086
 
 
   [JSON_KEYS({{json_doc[, _{{path}}_])}}|https://dev.mysql.com/doc/refman/5.7/en/json-search-functions.html#function_json-keys]
   
   Returns the keys from the top-level value of a JSON object as a JSON array, or, if a path argument is given, the top-level keys from the selected path. Returns NULL if any argument is NULL, the json_doc argument is not an object, or path, if given, does not locate an object. An error occurs if the json_doc_argument is not a valid JSON document or the path_ argument is not a valid path expression or contains a {} or * wildcard.
   
   The result array is empty if the selected object is empty. If the top-level value has nested subobjects, the return value does not include keys from those subobjects.
   
   Example SQL:
   ```sql
   SELECT JSON_KEYS(v) AS c1
   ,JSON_KEYS(v, 'lax $.a') AS c2
   ,JSON_KEYS(v, '$.b') AS c3
   ,JSON_KEYS(v, 'strict $.a[0]') AS c4
   ,JSON_KEYS(v, 'strict $.a[1]') AS c5
   FROM (VALUES ('{"a": [10, true],"b": {"c": 30}}')) AS t(v)
   LIMIT 10;
   ```
   Result:
   
   c1 | c2 | c3 | c4 | c5
   -- | -- | -- | -- | --
   ["a", "b"] | [NULL] | ["c"] | [NULL] | [NULL]
   
   

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