JSON¶

How to Create a JSON Table ?¶

You can create a JSON table using the CREATE TABLE statement. (For more information, please refer to Data Definition Language.) For example, please consider an example data as follows:

$ hdfs dfs -cat /table1/table.json
{ "title" : "Hand of the King", "name" : { "first_name": "Eddard", "last_name": "Stark"}}
{ "title" : "Assassin", "name" : { "first_name": "Arya", "last_name": "Stark"}}
{ "title" : "Dancing Master", "name" : { "first_name": "Syrio", "last_name": "Forel"}}

Tajo provides two ways to create a table for this data. First is a traditional way to create tables. Here is an example.

CREATE EXTERNAL TABLE table1 (
  title TEXT,
  name RECORD (
    first_name TEXT,
    last_name TEXT
  )
) USING JSON LOCATION '/table1/table.json';

With this way, you need to specify every column which they want to use. This will be a tedious work, and not appropriate for flexible JSON schema. Second is a simpler alternative to alleviate this problem. When you create an external table of JSON format, you can simply omit the column specification as follows:

CREATE EXTERNAL TABLE table1 (*) USING JSON LOCATION '/table1/table.json';

No matter which way you choose, you can submit any queries on this table.

> SELECT title, name.last_name from table1 where name.first_name = 'Arya';
title,name/last_name
-------------------------------
Assassin,Stark

Physical Properties¶

Some table storage formats provide parameters for enabling or disabling features and adjusting physical parameters. The WITH clause in the CREATE TABLE statement allows users to set those parameters.

The JSON format provides the following physical properties.

• text.delimiter: delimiter character. | or \u0001 is usually used, and the default field delimiter is |.
• text.null: NULL character. The default NULL character is an empty string ''. Hive’s default NULL character is '\\N'.
• compression.codec: Compression codec. You can enable compression feature and set specified compression algorithm. The compression algorithm used to compress files. The compression codec name should be the fully qualified class name inherited from org.apache.hadoop.io.compress.CompressionCodec. By default, compression is disabled.
• timezone: the time zone that the table uses for writting. When table rows are read or written, `timestamp` and `time` column values are adjusted by this timezone if it is set. Time zone can be an abbreviation form like ‘PST’ or ‘DST’. Also, it accepts an offset-based form like ‘UTC+9’ or a location-based form like ‘Asia/Seoul’.
• text.error-tolerance.max-num: the maximum number of permissible parsing errors. This value should be an integer value. By default, text.error-tolerance.max-num is 0. According to the value, parsing errors will be handled in different ways.
  • If text.error-tolerance.max-num < 0, all parsing errors are ignored.
  • If text.error-tolerance.max-num == 0, any parsing error is not allowed. If any error occurs, the query will be failed. (default)
  • If text.error-tolerance.max-num > 0, the given number of parsing errors in each task will be pemissible.
• text.skip.headerlines: Number of header lines to be skipped. Some text files often have a header which has a kind of metadata(e.g.: column names), thus this option can be useful.

The following example is to set a custom field delimiter, NULL character, and compression codec:

CREATE TABLE table1 (
  id int,
  name text,
  score float,
  type text
) USING JSON WITH('text.delimiter'='\u0001',
                  'text.null'='\\N',
                  'compression.codec'='org.apache.hadoop.io.compress.SnappyCodec');

Null Value Handling Issues¶

In default, NULL character in TEXT format is an empty string ''. In other words, an empty field is basically recognized as a NULL value in Tajo. If a field domain is TEXT, an empty field is recognized as a string value '' instead of NULL value. Besides, You can also use your own NULL character by specifying a physical property text.null.