Unhandled Query Planning Failure when working with a VALUES query with a column full of NULLs when there is also other processing happening.

[vogievetsky]

The SQL data loader in the web console uses a VALUES query to make a sample dataset for easy previewing. I know the description might make this sounds like a crazy corner case but in actuality this is a very common thing to stumble upon as in a sample of data (20 rows or so) with many column you are very likely to get a column that is all NULL.

Affected Version

All recent Druid versions that I tested including the latest build on master as of this writing and 30.0.0 RC.

Description

Here is a very simple (self contained) query that fails:

SELECT
  CAST("c1" AS VARCHAR) AS "channel",
  CAST("c2" AS VARCHAR) AS "cityName",
  PARSE_JSON("c3") AS "j"
FROM (
  VALUES
  ('ca', NULL, '{}'),
  ('de', NULL, '{"a":"1"}'),
  ('de', null, '{"a":"2"}')
) AS "t" ("c1", "c2", "c3")

All that is logged in the broker is:

2024-05-15T18:43:43,102 WARN [sql[8f567aa7-38c4-4d27-89dc-60189e66f5c9]] org.apache.druid.sql.http.SqlResource - Exception while processing sqlQueryId[8f567aa7-38c4-4d27-89dc-60189e66f5c9] (org.apache.druid.error.DruidException: Unhandled Query Planning Failure, see broker logs for details)

Which is not helpful

Curiously if one of the NULLs is changed to, say a '' it works:

Also if we remove the PARSE_JSON it also works:

From playing around with this it appears that there is some additional processing step that applying a function like PARSE_JSON adds that can not handle a NULL typed column that is being cast.