Why can't a COMMENT string in Postgres be concatenated?

Question

I tried to concatenate a longer comment over several lines to keep my code clean and was surprised to find that I can't. For example, the following returns a syntax error at the pipes:

COMMENT ON VIEW my_view IS 'foo' || 'bar';

Is there something special about comments?

Answer 1

The documentation explains about the argument:

The new comment, written as a string literal; or NULL to drop the comment.

So you cannot have an expression in that place, only a constant.

Fortunately the SQL standard has decreed that adjacent string literals be concatenated, so you can write

COMMENT ON VIEW my_view IS 'foo'
   'bar';

Answer 2

The documentation states that only text and no expressions are allowed:

COMMENT ON { ACCESS METHOD object_name | ... VIEW object_name } IS 'text'

Note the apos around text. But you may use dynamic created statements for that:

DO $$                      
BEGIN
    EXECUTE FORMAT('COMMENT ON VIEW myview IS %L', 'A very long ' || 'comment');
END
$$ LANGUAGE PLPGSQL;