Re: psql - add ability to test whether a variable exists

2017-08-26 8:54 GMT+02:00 Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>:

>
> Hello,
>
> As a follow-up to the \if patch by Corey Huinker, here is a proposal to
> allow testing whether a client-side variable exists in psql.
>
> The syntax is as ugly as the current :'var' and :"var" things, but ISTM
> that this is the only simple option to have a working SQL-compatible syntax
> with both client-side substitution and server side execution. See the
> second example below.
>

It is really ugly - the ? symbol is not used in pair usually - so it is
much more visible - it is bad readable.

Maybe some other syntax: :{fx xxx} .. where fx can be one from more
possible operators ? ! ...

>
> -- client side use
> psql> \set i 1
> psql> \if :?i?
> psql> \echo 'i is defined'
> psql> \endif

>
-- use server-side in an SQL expression
> psql> SELECT NOT :?VERSION_NUM? OR :'VERSION' <> VERSTION() AS bad_conf
> \gset
>

> psql> \if :bad_conf \echo 'too bad...' \quit \endif
>
> The other option would be to have some special keyword syntax, say
> "defined var", but then it would have to be parsed client side, and how to
> do that in an SQL expression is unclear, and moreover it would not look
> right in an SQL expression. If it would look like a function call, say
> "defined('var')", it would potentially interact with existing server-side
> user-defined functions, which is pretty undesirable. Hence the :?...?
> proposal above which is limited to variable subsitution syntax.

should not be solved by introduction \ifdef ?

>
> --
> Fabien.
>
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