`varchar(8000)` default in `FabricColumn` silently truncates strings

[sdebruyn]

Summary

FabricColumn.TYPE_LABELS maps the generic STRING type to VARCHAR(8000), and FabricColumn.string_type() / string_size() both default to 8000 when char_size is None. Fabric Warehouse supports varchar(MAX), so the 8000-character cap is an unnecessary hard limit that silently truncates any string column produced through these code paths.

Evidence (HEAD 0de2190, v1.10.0)

• dbt/adapters/fabric/fabric_column.py — TYPE_LABELS maps STRING → VARCHAR(8000); string_type(size) / string_size() fall back to 8000 when size is None.

These defaults flow into any code path that asks the adapter what type a string column should be without specifying a width — most commonly, models that copy long-text source columns (JSON payloads, free-text descriptions, serialized blobs) where the column type is inferred rather than declared in a contract.

User impact

• Any inferred-width string column is hard-capped at 8000 characters regardless of source data width. Long-text columns from a source — JSON payloads, free-text fields, serialized blobs — silently lose the tail beyond byte 8000 when materialized into a Fabric table.
• The truncation is silent: no warning, no error, just lost data. Users typically only notice when a downstream query returns visibly cut-off text.

Suggested fix

Default to varchar(MAX) (Fabric supports it) and only fall back to a fixed width when the user has explicitly requested one.

TYPE_LABELS = {
    ...
    "STRING": "VARCHAR(MAX)",
    "VARCHAR": "VARCHAR(MAX)",
    "NVARCHAR": "VARCHAR(MAX)",
}

@classmethod
def string_type(cls, size: int) -> str:
    if size is None or size <= 0:
        return "varchar(max)"
    return f"varchar({size})"

def string_size(self) -> int:
    if not self.is_string():
        raise DbtRuntimeError("Called string_size() on non-string field!")
    if self.char_size is None:
        return -1  # sentinel for MAX
    return int(self.char_size)

can_expand_to needs a small update so the -1 sentinel compares as the widest possible size.

A PR with the full change is linked from this issue.