Reading and Writing JSON Data in an Object Store

The PXF object store connectors support reading and writing JSON-format data. This section describes how to use PXF and external tables to access and write JSON data in an object store.

Note: Accessing JSON-format data from an object store is very similar to accessing JSON-format data in HDFS. This topic identifies object store-specific information required to read and write JSON data, and links to the PXF HDFS JSON documentation where appropriate for common information.

Prerequisites

Ensure that you have met the PXF Object Store Prerequisites before you attempt to read data from an object store.

Working with JSON Data

Refer to Working with JSON Data in the PXF HDFS JSON documentation for a description of the JSON text-based data-interchange format.

Data Type Mapping

Refer to Data Type Mapping in the PXF HDFS JSON documentation for a description of the JSON to Greenplum and Greenplum to JSON type mappings.

Creating the External Table

Use the <objstore>:json profile to read or write JSON-format files in an object store. PXF supports the following <objstore> profile prefixes:

Object Store	Profile Prefix
Azure Blob Storage	wasbs
Azure Data Lake	adl
Google Cloud Storage	gs
MinIO	s3
S3	s3

The following syntax creates a Greenplum Database external table that references JSON-format data:

CREATE [WRITABLE] EXTERNAL TABLE <table_name>
    ( <column_name> <data_type> [, ...] | LIKE <other_table> )
LOCATION ('pxf://<path-to-file>?PROFILE=<objstore>:json&SERVER=<server_name>[&<custom-option>=<value>[...]]')
FORMAT 'CUSTOM' (FORMATTER='pxfwritable_import'|'pxfwritable_export')
[DISTRIBUTED BY (<column_name> [, ... ] ) | DISTRIBUTED RANDOMLY];

The specific keywords and values used in the Greenplum Database CREATE EXTERNAL TABLE command are described in the table below.

Keyword	Value
<path‑to‑file>	The path to the directory or file in the object store. When the <server_name> configuration includes a pxf.fs.basePath property setting, PXF considers <path‑to‑file> to be relative to the base path specified. Otherwise, PXF considers it to be an absolute path. <path‑to‑file> must not specify a relative path nor include the dollar sign ($) character.
PROFILE=<objstore>:json	The PROFILE keyword must identify the specific object store. For example, s3:json.
SERVER=<server_name>	The named server configuration that PXF uses to access the data.
<custom‑option>=<value>	JSON supports the custom options described in the PXF HDFS JSON documentation.
FORMAT ‘CUSTOM’	Use FORMAT 'CUSTOM' with (FORMATTER='pxfwritable_export') (write) or (FORMATTER='pxfwritable_import') (read).

If you are accessing an S3 object store, you can provide S3 credentials via custom options in the CREATE EXTERNAL TABLE command as described in Overriding the S3 Server Configuration with DDL.

Read Example

Refer to Loading the Sample JSON Data to HDFS and the Read Example in the PXF HDFS JSON documentation for a JSON read example. Modifications that you must make to run the example with an object store include:

• Copying the file to the object store instead of HDFS. For example, to copy the file to S3:

  $ aws s3 cp /tmp/objperrow.jsonl s3://BUCKET/pxf_examples/
  

• Using the CREATE EXTERNAL TABLE syntax and LOCATION keywords and settings described above. For example, if your server name is s3srvcfg:

  CREATE EXTERNAL TABLE objperrow_json_s3(
    created_at TEXT,
    id_str TEXT,
    "user.id" INTEGER,
    "user.location" TEXT,
    "coordinates.values" INTEGER[]
  )
  LOCATION('pxf://BUCKET/pxf_examples/objperrow.jsonl?PROFILE=s3:json&SERVER=s3srvcfg')
  FORMAT 'CUSTOM' (FORMATTER='pxfwritable_import');
  

• If you want to access specific elements of the coordinates.values array, you can specify the array subscript number in square brackets:

  SELECT "coordinates.values"[1], "coordinates.values"[2] FROM singleline_json_s3;
  

Write Example

Refer to the Writing JSON Data in the PXF HDFS JSON documentation for write examples. Modifications that you must make to run the single-object-per-row write example with an object store include:

• Using the CREATE WRITABLE EXTERNAL TABLE syntax and LOCATION keywords and settings described above. For example, if your server name is s3srvcfg:

  CREATE WRITABLE EXTERNAL TABLE add_objperrow_json_s3(
    created_at TEXT,
    id_str TEXT,
    id INTEGER,
    location TEXT,
    coordinates INTEGER[]
  )
  LOCATION('pxf://BUCKET/pxf_examples/jsopr?PROFILE=s3:json&SERVER=s3srvcfg')
  FORMAT 'CUSTOM' (FORMATTER='pxfwritable_export');
  

• Using the CREATE EXTERNAL TABLE syntax and LOCATION keywords and settings described above to read the data back. For example, if your server name is s3srvcfg:

  CREATE EXTERNAL TABLE jsopr_tbl(
    created_at TEXT,
    id_str TEXT,
    id INTEGER,
    location TEXT,
    coordinates INTEGER[]
  )
  LOCATION('pxf://BUCKET/pxf_examples/jsopr?PROFILE=s3:json')
  FORMAT 'CUSTOM' (FORMATTER='pxfwritable_import');