`append` with null columns leads to runtime SQL error `UNION types x and y cannot be matched`

[Fanaen]

What happened?

• Doing an append in postgres with null columns on one side.
• prqlc outputs a query that leads to runtime failure UNION types integer and text cannot be matched.

PRQL input

prql target:sql.postgres

from invoices | select { an_id = invoice_id, a_date = null } | take 2
append (from employees | select { an_id = null, a_date = birth_date } | take 2)

SQL output

WITH table_0 AS (
  SELECT
    NULL AS an_id,
    birth_date AS a_date
  FROM
    employees
  LIMIT
    2
)
SELECT
  *
FROM
  (
    SELECT
      invoice_id AS an_id,
      NULL AS a_date
    FROM
      invoices
    LIMIT
      2
  ) AS table_1
UNION
ALL
SELECT
  *
FROM
  table_0

Expected SQL output

(
  SELECT
    invoice_id AS some_id,
    NULL AS some_date
  FROM
    invoices
  LIMIT
    2
)
UNION
ALL (
  SELECT
    NULL AS some_id,
    birth_date AS some_date
  FROM
    employees
  LIMIT
    2
)

MVCE confirmation

• Minimal example
• New issue

Anything else?

The SQL output works in the PRQL playground.
It seems some engines struggle to infer types with those SELECT * FROM in the way.

[vanillajonathan]

Maybe a note should added to the CHANGELOG file?