Lack of Nested and Implicit Data Support in Drill, PostgreSQL and Pentaho when Working with Json Data

JSon is great. It can contain a variety of data types in an expected format. It is becoming easier and easier to work with Json in existing formats as well making it a future workhorse for NoSQL based ETL. However, and not in the least because NoSQL ingestion needs to result in relational tables using SQL standards, there is still one bug to work out. Ingestion with Json will not break out nested tables and requires direct knowledge of data to complete tasks.

This may seem petty but when millions of recods are being read, it clearly is not.

In drill, this could potentially be overcome by creating a table for every single submap we wish to analyze but CREATE TABLE from the tool itself will not work. Instead, it is necessary to limit use cases to the data we want to use.

In PostgreSQL, it is possible to concatenate JSon data using a query whose individual results can then be saved. It is also possible to ‘pop’ keys that are unneeded. However, this approach requires many different tables at one per somewhat normalized form. It also requires recombining data.


SELECT row_to_json(r.*) FROM (SELECT nonJson AS nonJson, ((((data->'level1')::jsonb - 'bkey1')::jsonb - 'bkey2')::jsonb -'bkey3')::jsonb AS jdata FROM table WHERE data::jsonb ?| array['mustHaveKey'] AND data::jsonb ?| array['notHaveKey'] IS FALSE) r

Drill is still much more ahead of the game than Pentaho and PostgreSQL in terms of conversion though. Postgresql can guess types but has no function to attempt to automatically generate tables. Pentaho requires explicit conversion as well.

Of course, if one already knows every key that will be present, this is not a problem. That, however, means more maintenance as it is then impossible to write programs to automatically handle changes to data. Perhaps implicit conversion will happen soon but any argument as to data type issues should really look at the depth of the standards and conform to them instead of complaining.

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