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Hitachi Vantara Lumada and Pentaho Documentation

Hierarchical data

Pentaho supports a hierarchical data type (HDT) by means of the Pentaho EE Marketplace hierarchical data type plugin that adds the data type and creates five steps. These steps are designed to simplify string manipulation, with the ability to convert between HDT fields and formatted strings.

These steps improve performance on the steps that that treat hierarchical representations in string fields and enable you to address or modify nested array indices and keys. This data type can hold nested or complex data that is built by using objects and arrays and can also hold single elements. It can be used with any PDI step that uses hierarchical data.

The steps are:

Of the five steps, the first three steps have been created to manipulate generic hierarchical data and the last two steps work with data in the JSON format. The first two steps are used to create or modify hierarchical data, and the third step is used to deserialize the data. A common use for these steps is described in the following table:

Modify values from a single rowYou can take one row of data from an incoming step and then use that to either create a new hierarchical data type or modify a hierarchical data type that's on that incoming data stream.
Modify values from grouped rowsYou can take multiple rows of data, group them based on a common field or set of fields, and then create a new hierarchical data type or modify and existing data type.
Extract to RowsYou can take data out of the hierarchical data type and place it onto the PDI data stream and use the data with other steps.

The last two steps are specifically for working with JSON and JSON Lines (JSONL) files. Differences between the Hierarchical JSON input step and the JSON Input step are:

  • The JSON input step uses the JSONPath path specification, the Hierarchical JSON input step uses an HDT path specification.
  • The JSON input step reads the entire JSON file into memory, the Hierarchical JSON input step processes the JSON as an input stream and loads only the specified content . You can load filtered subsets of large JSON files, or separate lines from a JSONL directly into a HDT field for extraction or modification by other steps.
  • The JSON input step loads content onto multiple fields of a stream, the Hierarchical JSON input step has filters you can apply to fetch the subset of a JSON file.
Hierarchical JSON inputYou can read an entire JSON file into memory and send it to a single HDT field on the steps output field. You can use the Split rows across path option on the Output tab to extract each JSON parent to a row, allowing you to process the data in blocks and reduce the amount of memory needed for processing the data.
Hierarchical JSON OutputYou can convert hierarchical data to a JSON file and write it to an output field or pass the output to a PDI servlet.

You can convert data between JSON and HDT types with the Type and Format field options in steps that support HDT. In the following example, the Select values step is set to convert hierarchical data to JSON: