[PL/PgSQL] error checks vs error trapping

Hi all.
It's not clear to me whether it's faster to check error conditions
before actually executing some
statements or to add error trapping code to "react" to errors.

The documentation states that the former should be faster than the
latter. Which is more or less
the same as I can experience in C++ with try/catch constructs.
But from time to time I read something on this very mailing list that
makes me think the reality is
different.

Is there any authoritative, and possibly proven, answer to this topic?

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Vincenzo Romano at NotOrAnd Information Technologies
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