add constraints to views

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Please consider the following situation.

CREATE TABLE test_table (
~ id SERIAL PRIMARY_KEY,
~ tag BOOLEAN,
~ field1 INTEGER
);

CREATE VIEW test_view (
~ SELECT * FROM test_table WHERE tag
);

Now I want ad a NOT NULL constraint to the view on field1. I tryed the
following, but neither works.

ALTER TABLE test_view ADD CONSTRAINT isit CHECK ( field1 IS NOT NULL);
ALTER TABLE test_view ALTER field1 SET NOT NULL;

How can I do this?

Regards

Andreas
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