Thanks!

The change from the string NULL to the string "NULL" broke a test in Ibis. What was the rationale for this change?

I see

One notable change (which we might want to find a better solution for):
the SQLNULL type ToString used to return NULL, now turned into "NULL", because when it appears in a query it has to be quoted.
See test/sql/types/test_null_type.test

In particular, not 100% following this:

because when it appears in a query it has to be quoted.

Wouldn't that be true for all the other types as well? Why would NULL need to be special cased, the return value of typeof already produces the string with characters NULL.

Quotes are added to other types that need them as well - for example:

D select typeof(null::struct(this_is_an_unquoted_identifier integer)) AS struct_type;
┌────────────────────────────────────────────────┐
│                  struct_type                   │
│                    varchar                     │
├────────────────────────────────────────────────┤
│ STRUCT(this_is_an_unquoted_identifier INTEGER) │
└────────────────────────────────────────────────┘

D select typeof(null::struct("this is a quoted identifier" integer)) AS struct_type;
┌───────────────────────────────────────────────┐
│                  struct_type                  │
│                    varchar                    │
├───────────────────────────────────────────────┤
│ STRUCT("this is a quoted identifier" INTEGER) │
└───────────────────────────────────────────────┘

The main idea here is that LogicalType::ToString can be round-tripped within our parser, i.e. you can take the output of that and use it as a type directly in SQL. This is not particularly important for typeof - but is used elsewhere in the code to turn our logical operators back into SQL strings.

We could strip the quotes from NULL for typeof specifically if it is a big issue?

To add to that - here's the SQL that shows that NULL needs to be quoted to be used as a type, but INT does not:

D select cast(null as int) AS result;
┌────────┐
│ result │
│ int32  │
├────────┤
│   NULL │
└────────┘
D select cast(null as null) AS result;
-- Parser Error: syntax error at or near "null"
-- LINE 1: select cast(null as null) AS result;
--                             ^
D select cast(null as "null") AS result;
┌────────┐
│ result │
│ int32  │
├────────┤
│   NULL │
└────────┘

Ah, got it. The double quote wasn't clicking, but now it makes sense: you need them to differentiate betwee NULL-the-value and NULL-the-identifier-that-refers-to-a-type.