MaxCompute:Instructions for using JSON data types in MaxCompute

Background information

Semi-structured data is a mix of structured data and unstructured data. Semi-structured data has flexible schemas that do not have strong constraints. In most cases, the schemas of semi-structured data are self-descriptive. JSON data is typical semi-structured data. To strengthen the support for semi-structured data in MaxCompute SQL, MaxCompute supports items such as schema evolution, the JSON STRING type, built-in complex type functions, and Lambda expressions. However, you must process semi-structured data in a standardized manner and import the processed data to tables that have schemas and are used to store structured data. If your business data changes, you must use the schema evolution feature to explicitly execute DDL statements to modify table schemas.

The preceding mode has strong constraints on schemas. In this mode, you cannot quickly import semi-structured data to a system. Data whose schemas do not conform to schema specifications can only be discarded during data import. To resolve this issue, the MaxCompute development team designs JSON data types. The JSON data types are suitable for semi-structured data that does not have strong constraints on schemas and allow you to fully utilize the column-oriented storage mode. This helps meet requirements for high flexibility and high performance.

Basic principles

JSON data types are new data types introduced in MaxCompute. You can use JSON data types in a similar way as other data types. The support for JSON data types frees you from managing schemas. After you insert JSON data into a MaxCompute table, MaxCompute automatically extracts a public schema and optimizes the schema. To improve performance, we recommend that you store JSON data in column-oriented storage mode. The following code shows an example:

CREATE TABLE json_table
(
    json_val  json
);

CREATE TABLE string_table
(
    string_val  STRING
);

INSERT INTO string_table VALUES
        ('{"a":1, "b":2}')
        ,('{"a":"key", "b":2}')
        ,('{"c":3}');

INSERT INTO json_table
SELECT  json_parse(string_val)
FROM    string_table;

When you insert data into a MaxCompute table, MaxCompute automatically extracts a public schema <"a":binary, "b":bigint, "c":bigint>. When you query data from the table, MaxCompute can prune columns based on the public schema. This reduces the amount of data that needs to be read and improves query efficiency. The following code shows a query example.

SELECT  json_val["b"]
        ,json_val["c"]
FROM    json_table
;  
-- When you query data from the table, MaxCompute performs a column pruning operation and retains only the b and c variables.
+-----+-----+
| _c0 | _c1 |
+-----+-----+
| 2   | NULL |
| 2   | NULL |
| NULL | 3   |
+-----+-----+

MaxCompute stores data that does not belong to the public schema by using the BINARY type instead of the original STRING type. This can reduce the storage space that is required. Compared with user-defined functions (UDFs), JSON data types help improve the efficiency of conversions between the STRING type and a JSON data type.

Limits

• The supported development tools include the MaxCompute client (odpscmd), MaxCompute Studio, and DataWorks. External ecosystems such as Dataphin are not supported. If you need to use JSON data types supported by MaxCompute in an external ecosystem, you must check whether the external ecosystem supports the JSON data types. The following table describes the items that you must take note of when you use the MaxCompute client (odpscmd) or MaxCompute Studio as the development tool for data of a JSON type.

  MaxCompute client (odpscmd)	MaxCompute Studio
  You must upgrade the MaxCompute client to V0.46.5 or later. Otherwise, you cannot execute the desc json_table statement or download data of JSON types by using Tunnel. You must set the use_instance_tunnel parameter to false. This parameter is included in the odps_config.ini file in the conf folder of the installation path of the client. Otherwise, an error is reported when you perform a query.	MaxCompute Studio allows you to query JSON data. MaxCompute Studio does not allow you to upload or download JSON data.

• If you want to use another engine, such as Hologres, to read data from a MaxCompute table, JSON data in the table cannot be read.

• Columns of a JSON type cannot be added to a MaxCompute table.

• You are not allowed to compare data of a JSON type with data of other types, execute SQL statements that contain the ORDER BY or GROUP BY clause on data of a JSON type, or use columns of a JSON type as join keys.

• For data of the JSON NUMBER type, the integer part is stored by using the BIGINT type, and the decimal part is stored by using the DOUBLE type. If the integer part exceeds the range that is supported by the BIGINT type, an integer overflow occurs. When the decimal part is converted into the DOUBLE type, precision loss occurs.

• Strings that are used to generate JSON data cannot contain \u0000, which is the null character in Unicode.

• Java UDFs and Python UDFs do not support JSON data types.

• Clustered tables cannot store JSON data.

• SDKs for Java in versions earlier than V0.44.0 and PyODPS in versions earlier than V0.11.4.1 do not support JSON data types

Literal constants

JSON data types are defined based on JSON standards and include BOOLEAN, NUMBER, STRING, NULL, ARRAY, and OBJECT. For data of the JSON NUMBER type, different parts are separately stored by using the BIGINT type and DOUBLE type. If a part of data of the JSON NUMBER type exceeds the specified range, precision loss occurs. JSON 'null' is different from SQL null.

JSON 'null'
JSON '123'
JSON '123.34'
JSON 'true'
JSON '{"id":123,"name":"MaxCompute"}'
JSON '[12, 34]'

The constants that you use must conform to JSON standards. For example, JSON '{id:123,"name":"MaxCompute"}' is an invalid JSON string. id must be enclosed in double quotation marks (").

Define JSON data types

You can create a table that contains data of a JSON type in the same way as you create a table that contains data of basic data types. You do not need to specify schemas.

CREATE TABLE mf_json_table (json_val JSON);

Generate JSON data

You can use one of the following methods to generate JSON data:

• JSON Literal

  insert into mf_json_table values (json '123');

• JSON functions

  -- json_object and json_array are built-in functions of MaxCompute.
  insert into mf_json_table select json_object("key",123, "value", "abc");
  
  select * from mf_json_table;
  
  -- The following result is returned:
  +----------+
  | json_val |
  +----------+
  | 123      |
  | {"key":123,"value":"abc"} |
  +----------+
  
  
  insert into mf_json_table select json_array("key",234, "value", "abc");
  
  select * from mf_json_table;
  
  -- The following result is returned:
  +----------+
  | json_val |
  +----------+
  | 123      |
  | ["key",234,"value","abc"] |
  | {"key":123,"value":"abc"} |
  +----------+

• Type conversion

  You must take note of the differences between the cast function and the json_parse function. For more information, see Complex type functions.

  insert into mf_json_table select cast("abc" as json);
  select * from mf_json_table;
  -- The following result is returned:
  +----------+
  | json_val |
  +----------+
  | 123      |
  | "abc"    |
  | ["key",234,"value","abc"] |
  | {"key":123,"value":"abc"} |
  +----------+

Access JSON data

You can use the indexing method, json_extract function, or get_json_object function to access JSON data. The returned results are of a JSON type.

-- 123 is returned.
SELECT v['id'] 
  FROM VALUES (JSON '{"id":123}') as t(v);
  
-- 12 is returned.
SELECT v[0] 
  FROM VALUES (JSON '[12, 34]') as t(v);
  
-- 1 is returned.
select v['x']['a']  from values (json '{"x": {"a": 1, "b": 2}}') as t(v);

-- NULL is returned.
SELECT v[0] 
FROM VALUES (JSON '{"id":123}') as t(v);

-- NULL is returned.
SELECT v['not_exists'] 
FROM VALUES (JSON '{"id":123}') as t(v);

-- Access JSON data by using the get_json_object function. MaxCompute is returned.
SELECT GET_JSON_OBJECT(v, '$.x.name')
  FROM VALUES (JSON '{"x": {"id": 1001, "name": "MaxCompute"}}') as t(v);
  
-- The following result is returned:
+-----+
| _c0 |
+-----+
| MaxCompute |
+-----+

-- Access JSON data by using the json_extract function. "MaxCompute" of a JSON type is returned.
SELECT JSON_EXTRACT(v, '$.x.name') 
  FROM VALUES (JSON '{"x": {"id": 1001, "name": "MaxCompute"}}') as t(v);
  
-- The following result is returned:
+-----+
| _c0 |
+-----+
| "MaxCompute" |
+-----+