Re: PSQL commands: \quit_if, \quit_unless

On Sat, Dec 3, 2016 at 2:16 AM, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> wrote:
>> My guess is that something comparable to where pgbench is would be a
>> reasonable target --- not least because I think we should strive to
>> reduce unnecessary differences between psql and pgbench metalanguages.
>>
>> I'm not sure that I'm ready to propose that they should share the same
>> expression engine, but perhaps it's not a totally wacky idea.
>
>
> I'm trying to summarize a proposal for a conditional structure:
>
> - existing psql ":"-variables can be used, allowing anything from SQL
> (eg querying about available objects, features, extensions,
> current settings...)
>
> - target psql conditional syntax could be:
>
> \if <expression>
> ...
> \elif <...>
> ...
> \else
> ...
> \endif
>
> - possible incremental implemention steps on this path:
>
> (1) minimal condition and expression, compatible with
> a possible future full-blown expression syntax
>
> \if :variable
> \if not :variable -- maybe \if ! :variable
> ...
> \endif
>
> (2) add "\else"
>
> (3) add "\elif ..." (or maybe "\elsif ..."?)
>
> (4) add greater but limited expressions, compatible with a full blown
> expression syntax (eg \if :var/const <comparison-operator> :var/const)
>
> (5) add full-blown <expression> support for \if, which suggest that
> it would also be available for \set
>
>
> Does this looks okay, or does it need to be amended?
>
> A few comments:
>
> Given the experience with pgbench and the psql context, I do not think that
> it would really need to go beyond step 2 above, but I agree that I may be
> wrong and it is best to be prepared for that from the start. Given the
> complexity and effort involved with (5), it seems wise to wait for a clearer
> motivation with actual use-cases before going that far.

Well, my vote would be to go all the way to #5 in one commit.
Stopping short of that doesn't seem to me to save enough work to make
much sense. I don't think we're talking about anything all that
complex, and it will make future improvements a lot simpler.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company