Обсуждение: argument of AND must not return a set when using regexp_matches

I've been getting a funny SQL error, which I've boiled down to this case.

SELECT (regexp_matches('abc', '(.)b(.)'))[1] IS NOT NULL
-- Returns true, as expected

SELECT (regexp_matches('abc', '(.)b(.)'))[1] IS NOT NULL AND true
-- Gives this error:
ERROR: argument of AND must not return a set
SQL state: 42804

Can anyone make heads or tails of it? Is it a real bug? Is there a work around?

Postgres 8.3

Robert James <srobertjames@gmail.com> writes:
> SELECT (regexp_matches('abc', '(.)b(.)'))[1] IS NOT NULL AND true
> -- Gives this error:
> ERROR: argument of AND must not return a set
> SQL state: 42804

> Can anyone make heads or tails of it? Is it a real bug? Is there a work around?

It's not a bug: regexp_matches returns a set of rows, not a scalar
result.

You might want to stick it into a sub-select as per the trick suggested
in the manual - then you get a NULL rather than zero rows when there's
no match.  (Or, in the particular example at hand, it's not very clear
why you're using regexp_matches at all and not a plain old ~ operator.)

            regards, tom lane