Constraints, Checks and Default Values

The Airport extention for DuckDB supports tables that have constraints and checks. The Arrow Flight server must declare these constraints and checks in the Apache Arrow schema of the table.

Not Null Constraints

If the Apache Arrow specifies that a field is not nullable, that is interpreted as a “not null” constraint being applied to the table. This constraint will be validated when the table is used in an INSERT or UPDATE statement.

Checks

Apache Arrow does not actually have a formalized method to serialize checks that may exist on a table, as such the checks will be serialized using msgpack on the Apache arrow schema.

Checks are specified a set of string based expressions. An example check expression could be "color in ('blue', 'green')", that would restrict the value of the color field to be either “blue” or “green”. These checks are serialized using this msgpack structure, then stored in a metadata key called check_constraints:

struct AirportTableCheckConstraints {
  std::vector<std::string> constraints;

  MSGPACK_DEFINE_MAP(constraints)
};

Default Values

In an Arrow Flight table there is the ability for a default value to be specified for the column when rows are inserted. Apache Arrow doesn’t have a way to specify this, so a metadata field is used on the field in the Arrow schema.

If there is a metadata field named default specified on a field, its string value is parsed as a SQL expression that will be used to produce the default value for that field. If the expression is not able to be parsed the table will not be created.