Re: Desirability of client-side expressions in psql?

2018-03-03 11:35 GMT+01:00 Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>:

>
> Hello devs,
>
> This is a discussion without actual patch intended for pg12, to be added
> to CF 2018-09. The expected end result is either "returned with feedback",
> meaning proceed to send some big patch(es), or "rejected", meaning the
> project does not want this, no point in submitting something.
>
> Client "psql" has an "\if" which can test a boolean value and has
> ":"-prefixed variables, including special presets such as ":VERSION_NUM"
> and ":SERVER_VERSION_NUM".
>
> The features are already usable because one can do server-side expressions
> (if connected), which is a little cumbersome and ugly but nevertheless
> functional, eg:
>
> SELECT :VERSION_NUM = :SERVER_VERSION_NUM AS "same_version" \gset
> \if :same_version
> ...
>
> However, when the "\if" patch was discussed, there was the underlying idea
> to extend psql so as to add client-side expression. That would allow things
> like:
>
> \let i <some arithmetic or logical expression...>
> \if :VERSION_NUM = :SERVER_VERSION_NUM
> ...
>
> Before eventually starting on this subject with a loose objective of
> targeting 12.0, I would like to ascertain, especially from committers, but
> also from others, that:
>
> (1) the objective is desirable (i.e. avoid ending with "we do not want
> this feature on principle, the cost-benefit balance is not good
> enough").
>
> (2) maybe have a feedback on the proposed changes (not necessarily
> distinct patches, opinions are welcome), which would be to:
> (a) extend pgbench expressions so that they can handle what
> psql can do (eg variable-exists test which are available in psql)
> (b) maybe do some preliminary refactoring (eg create
> "pgbench/expression.c", "pgbench/variable.c")
> (c) move the pgbench expression engine to fe-utils
> (lexer, parser, execution...),
> (d) do some renaming (eg some "feex" prefix for "front-end
> expressions" to the various functions & types?),
> (e) abstract pgbench and psql variables so that they can be used
> transparently by expressions (i.e. some API alignment)
> (f) connect the engine to "psql"
> (g) create a shared documentation about these expressions,
> referenced from both psql and pgbench documentations.
> (h) provide non-regression tests on psql side as well.
>
> The overall transformation would be quite large (about 2000 lines moved
> around) but "quite" simple (it is moving an existing, working and tested
> feature to allow using it elsewhere), not a lot of new code per se.
>

I understand the request of some simple expression evaluation for pgbench
and conditional execution for psql. Because syntax is same, then share code
is really good idea. Lexer, parser, variable processing should be moved to
fe-utils, other implemented on place. We don't need all commands of pgbench
in psql, and we don't need interactive loop and psql commands in pgbench.
But the syntax of input commands is same on both environments, and all on
this level can be shared via some library. Some shared commands can be
implemented in other library, and called from final positions.

Using some simple expressions evaluations is much more simple, then
integration some full functional VM like lua, Python - and still good
enough.

I have not the feedback from psql users about missing strong integrated
language. What is current weak place of psql is tab complete and readline
multiline editor. The pgcli is better - and second hand, everything else is
better in psql.

Regards

Pavel

>
> --
> Fabien.
>
>