Allow null values in multi-dimensional arrays

[antiguru]

Allow constructing multi-dimensional arrays with null values instead of panicking. Similarly to PostgreSQL, treat null elements as zero-dimensional arrays.

Fixes MaterializeInc/database-issues#9757

[mgree]

This should likely come with changes to is_instance_of and is_instance_of_sql (and possibly the Datum Arbitrary instance). (Assuming I understand this as a new behavior correctly. Worth a cross check no matter what!)

[antiguru]

This should likely come with changes to is_instance_of and is_instance_of_sql (and possibly the Datum Arbitrary instance). (Assuming I understand this as a new behavior correctly. Worth a cross check no matter what!)

I understand but I don't know what needs to be true for an array datum to be instances of another a specific array type. Do we need to consider the dimensions?

[antiguru]

We might need another error variant to be precise, but I feel we're good with the current one being used in two different contexts (length vs. dims).

[petrosagg]

Postgres also checks for the lower bound being the same, even if the lengths are the same and it also produces a more generic error message than us:

postgres=# SELECT ARRAY['[0:0]={1}'::int[], '[1:1]={2}'::int[]];
ERROR:  multidimensional arrays must have array expressions with matching dimensions

What do you think of changing our error variant to give the same generic error message and then here we just do a simple if actual_dims != expected_dims { ... } ?

[mgree]

This should likely come with changes to is_instance_of and is_instance_of_sql (and possibly the Datum Arbitrary instance). (Assuming I understand this as a new behavior correctly. Worth a cross check no matter what!)

I understand but I don't know what needs to be true for an array datum to be instances of another a specific array type. Do we need to consider the dimensions?

See is_instance_of_sql:

materialize/src/repr/src/scalar.rs

Lines 1198 to 1211 in 362c175

	(Datum::Array(array), SqlScalarType::Array(t)) => {
	array.elements.iter().all(|e| match e {
	Datum::Null => true,
	_ => is_instance_of_scalar(e, t),
	})
	}
	(Datum::Array(array), SqlScalarType::Int2Vector) => {
	array.dims().len() == 1
	&& array
	.elements
	.iter()
	.all(|e| is_instance_of_scalar(e, &SqlScalarType::Int16))
	}
	(Datum::Array(_), _) => false,

It seems the dimensions don't matter (unless you're dealing with int2vec weirdness).