Case statement with one branch being null gives "Mismatch between schema and batches"

[mslapek]

Describe the bug

Case statement, where the first branch is NULL and another one is a number causes "Mismatch between schema and batches" error.

In batch schema, such column has Int32 datatype (or some similar).

In plan schema, such columns have an invalid datatype Null.

To Reproduce

Add this to the middle of pg_compat_union.slt:

statement ok
CREATE TABLE aggregate_test_100_nullable_by_sql AS
SELECT
    *,
    CASE
        WHEN c4 % 3 = 0 THEN NULL
        ELSE c5
    END AS n5,
    CASE
        WHEN c3 % 3 != 0 THEN c9
        ELSE NULL
    END AS n9
FROM aggregate_test_100_by_sql

Under Postgres this statement succeeds.

Expected behavior

To have the same behavior as Postgres: success.

Additional context

No response

[mslapek]

As a workaround you can add CAST to the first branch of CASE:

CREATE TABLE aggregate_first_50_by_sql AS
SELECT
    *,
    CASE
        WHEN c4 % 3 = 0 THEN CAST(NULL AS INT)
        ELSE c5
    END AS n5
FROM aggregate_test_100_by_sql