Re: psql \set vs \copy - bug or expected behaviour?

On 17/08/12 18:38, Tom Lane wrote:
> Bruce Momjian<bruce@momjian.us>  writes:
>> On Fri, Oct 21, 2011 at 05:31:41PM -0400, Robert Haas wrote:
>>> I'm not sure whether that's a bug per se, but I can see where a
>>> behavior change might be an improvement.
>
>> I did some research on this and learned a little more about flex rules.
>
>> Turns out we can allow variable substitution in psql whole-line
>> commands, like \copy and \!, by sharing the variable expansion flex
>> rules with the code that does argument processing.
>
> Well, it'd be nice to allow substitution there ...
>
>> What we can't easily do is to allow quotes to prevent variable
>> substitution in these whole-line commands because we can't process the
>> quotes because that will remove them.
>
> ... but if there is then no way to prevent it, that's absolutely
> unacceptable.

If I'm understanding this correctly, \copy parsing just passes the query 
part unaltered as part of a COPY statement back into the top-level 
parser. Likewise with the \!shell stuff (but presumably to execve).

To handle variable-substitution correctly for \copy we'd need to 
duplicate the full parsing for COPY. For \! we'd need something which 
understood shell-syntax (for the various shells out there). Ick.

Or you'd need a separate variable-bracketing {{:x}} syntax that could 
work like reverse dollar-quoting. Also Ick.

As far as we know this has only inconvenienced one person (me) badly 
enough to report a maybe-bug. Thanks for trying Bruce, but I fear this 
is one itch that'll go unscratched.

Rest assured I'm not about to storm off and replace all my installations 
with MySQL :-)

--   Richard Huxton  Archonet Ltd